Billy Sharp scored twice as Southampton took another step towards automatic promotion to the Premier League.
Jos Hooiveld opened the scoring with a header from a corner before Sharp rifled into the roof of the net.
Sharp then added his second from close range after Adam Lallana picked him out at the back post with a superb pass.
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Southampton had been in the top flight for 27 years before they were relegated in 2005.
Tommy Rowe hit a late consolation for the hosts but the win leaves the Saints five points clear of third-placed West Ham with two games remaining.
Nigel Adkins' side, who host struggling Coventry on the final day, know they can secure automatic promotion as soon as Saturday when they head to Middlesbrough.
They took control from the start, with Hooiveld beating Paul Jones from Dan Fox's corner for his seventh of the season.
"Southampton were thoroughly professional. Nigel Adkins will congratulate his players for ticking this match off and they are one step closer to promotion to the Premier League. They were hurt last week with the defeat to Reading, but they dusted themselves off and played well."
Five minutes later they were two ahead when Lallana managed to stay on his feet and cross for Richard Chaplow, who teed up Sharp to thump in.
After standing firm against Peterborough's attempts to respond, the Saints made it three just before the hour.
Rickie Lambert's cross-shot fell for Sharp, who guided in for his 17th of the season and his sixth in six games.
Rowe did manage to pull one back when he converted from six yards from a Tyrone Barnett cross.
It was a third straight defeat for Posh, but results elsewhere meant they are guaranteed to be playing second-tier football next season.
Southampton boss Nigel Adkins: "It was a good victory away from home and the players deserve great credit for that.
"It was a thoroughly professional display and an excellent response to the defeat against Reading on Friday.
"We are not concerning ourselves with results elsewhere or the amount of points we may need to secure promotion.
"We have two matches left and we want to put six more points on the board. Our aim has always been to be champions and that hasn't changed."
Peterborough boss Darren Ferguson on securing Championship safety: "It is our biggest achievement as a club by miles.
"But it is a very different feeling to winning promotion. This is more about relief than joy.
"We have stayed up because we were outstanding in the first half of the season.
"We haven't been great lately but always knew the effects of having such a short break after winning the League One play-offs last summer would catch up with us."
- 28 Jones
- 05 Zakuani
- 18 Brisley
- 27 Alcock
- 14 Rowe
- 17 Newell (McCann - 72' )
- 23 Frecklington (Little - 76' )
- 08 Tomlin
- 10 Boyd
- 20 Barnett
- 26 Taylor
- 01 Lewis
- 02 Little
- 29 Ntlhe
- 11 McCann
- 09 Ball
- 01 Davis
- 06 Fonte
- 12 Butterfield
- 13 Fox
- 26 Hooiveld (Martin - 63' )
- 08 Cork
- 14 Hammond
- 18 Chaplow
- 20 Lallana
- 07 Lambert
- 11 Sharp (Guly - 87' )
- 02 Richardson
- 03 Harding
- 16 Martin
- 10 Guly
- 33 De Ridder
Live Text Commentary
Full Time The referee signals the end of the game.90:00 +3:29
Effort from outside the area by Richard Lambert goes wide right of the goal.90:00 +2:21
Guly Do Prado has an effort at goal from outside the box which misses left.90:00 +0:54
Paul Taylor is caught offside. Kelvin Davis takes the free kick.86:24
Free kick taken by Daniel Fox.86:24 Substitution
Substitution (Southampton) makes a substitution, with Guly Do Prado coming on for Billy Sharp.86:24
Handball by Grant McCann.85:18
The assist for the goal came from Tyrone Barnett.85:18 Goal scored
Goal! - Tommy Rowe - Peterboro 1 - 3 Southamp'n Tommy Rowe grabs a goal from inside the six-yard box to the bottom left corner of the goal. Peterborough 1-3 Southampton.84:33
Paul Taylor takes a shot. Save made by Kelvin Davis. Inswinging corner taken from the right by-line by Grant McCann.84:22
Tommy Rowe takes a shot. Comfortable save by Kelvin Davis.82:44
Grant McCann has a shot on goal from outside the box which misses left.78:51
The assistant referee flags for offside against Billy Sharp. Gabriel Zakuani restarts play with the free kick.78:36
Mark Little crosses the ball, clearance made by Jack Cork.77:21
Lee Tomlin crosses the ball, clearance made by Jose Fonte. Corner taken by Lee Tomlin from the left by-line, save made by Kelvin Davis.74:30
Direct free kick taken by Shaun Brisley.74:30 Substitution
Substitution Mark Little joins the action as a substitute, replacing Lee Frecklington.74:30
Richard Lambert gives away a free kick for an unfair challenge on Craig Alcock.73:40
The ball is swung over by Richard Chaplow, comfortable save by Paul Jones.72:00 Substitution
Substitution Joe Newell goes off and Grant McCann comes on.
Outswinging corner taken by Paul Taylor from the right by-line, Dean Hammond makes a clearance.69:53
Paul Taylor takes a shot. Save made by Kelvin Davis.68:14
Dean Hammond takes a shot. George Boyd gets a block in. Corner taken by Adam Lallana from the left by-line to the near post, clearance made by Gabriel Zakuani.67:36
Richard Chaplow takes a shot. Save by Paul Jones.67:13
Richard Chaplow crosses the ball, Lee Frecklington makes a clearance.65:42
The ball is sent over by Adam Lallana.65:18
Foul by Shaun Brisley on Richard Lambert, free kick awarded. Direct free kick taken by Daniel Fox.64:35
Paul Taylor takes a shot. Kelvin Davis makes a comfortable save.63:06
The ball is sent over by Danny Butterfield, Gabriel Zakuani makes a clearance. Inswinging corner taken left-footed by Daniel Fox, Headed effort from inside the area by Jose Fonte misses to the right of the goal.62:20 Substitution
Substitution Aaron Martin joins the action as a substitute, replacing Jos Hooiveld.60:56
Lee Tomlin delivers the ball, Billy Sharp manages to make a clearance.56:58
Assist on the goal came from Adam Lallana.56:58 Goal scored
Goal! - Billy Sharp - Peterboro 0 - 3 Southamp'n Billy Sharp grabs a goal from inside the box to the bottom right corner of the goal. Peterborough 0-3 Southampton.56:13
Tyrone Barnett is flagged offside by the assistant referee. Kelvin Davis takes the indirect free kick.53:19
The ball is sent over by Lee Tomlin, blocked by Adam Lallana. Corner taken by George Boyd, Headed effort from inside the area by Tyrone Barnett misses to the left of the goal.52:58
Short corner taken by George Boyd from the right by-line.52:39
Richard Chaplow gives away a free kick for an unfair challenge on Tommy Rowe. George Boyd takes the direct free kick.51:49
Tyrone Barnett gives away a free kick for an unfair challenge on Jos Hooiveld. Jose Fonte takes the direct free kick.49:44
Inswinging corner taken from the left by-line by Adam Lallana, Tommy Rowe manages to make a clearance.48:42
Inswinging corner taken from the right by-line by Jos Hooiveld, clearance made by Gabriel Zakuani.47:56
The ball is delivered by Lee Tomlin, Shaun Brisley takes a shot. Kelvin Davis makes a save.47:09
Free kick awarded for a foul by Dean Hammond on Lee Tomlin. George Boyd takes the free kick.45:01
The referee gets the second half started.45:00 +3:07 Half time
Half Time The first half comes to an end.45:00 +0:59
Jos Hooiveld concedes a free kick for a foul on Tyrone Barnett. Direct free kick taken by George Boyd.43:16
Free kick awarded for a foul by Gabriel Zakuani on Danny Butterfield. Free kick taken by Kelvin Davis.42:09
George Boyd takes a shot. Blocked by Richard Chaplow. George Boyd takes a inswinging corner to the near post, clearance made by Jose Fonte. Lee Tomlin produces a cross, Jos Hooiveld makes a clearance.40:03
The ball is sent over by Craig Alcock, clearance made by Jos Hooiveld.39:59
The ball is sent over by Craig Alcock, Dean Hammond manages to make a clearance.39:34
Foul by Richard Chaplow on Joe Newell, free kick awarded. George Boyd takes the free kick.38:48
Billy Sharp fouled by Gabriel Zakuani, the ref awards a free kick. Jose Fonte takes the direct free kick.38:18
A cross is delivered by Daniel Fox, clearance by Lee Frecklington.37:19
Danny Butterfield produces a cross, Craig Alcock manages to make a clearance.36:14
The ball is crossed by Danny Butterfield, clearance made by Lee Frecklington.35:10
Craig Alcock concedes a free kick for a foul on Adam Lallana. Free kick taken by Daniel Fox.32:27
Foul by Tyrone Barnett on Jose Fonte, free kick awarded. Kelvin Davis restarts play with the free kick.29:57
The offside flag is raised against Billy Sharp. Paul Jones takes the free kick.29:13
Inswinging corner taken from the right by-line by Daniel Fox, Gabriel Zakuani manages to make a clearance.25:17
The ball is sent over by Danny Butterfield, clearance by George Boyd.24:11
A cross is delivered by Paul Taylor, Jos Hooiveld manages to make a clearance.23:41
Centre by Lee Tomlin, clearance by Jos Hooiveld.22:54
Shot from just outside the box by Shaun Brisley misses to the right of the target.22:14
Inswinging corner taken from the right by-line by Daniel Fox, Gabriel Zakuani makes a clearance.22:05
The ball is delivered by Billy Sharp, clearance by Gabriel Zakuani.21:16
Corner taken by Adam Lallana, Tyrone Barnett makes a clearance.21:09
Danny Butterfield sends in a cross, clearance by Craig Alcock.21:04
The ball is swung over by Richard Chaplow, Gabriel Zakuani manages to make a clearance.20:01
Gabriel Zakuani produces a right-footed shot from deep inside the penalty box which goes wide of the left-hand upright.19:35
Corner from left by-line taken by Tyrone Barnett.18:56
The ball is swung over by Paul Taylor, Danny Butterfield manages to make a clearance. Inswinging corner taken right-footed by Lee Tomlin from the left by-line, Gabriel Zakuani takes a shot. Save by Kelvin Davis.16:15
Corner taken right-footed by Lee Tomlin to the near post, Jos Hooiveld makes a clearance.16:08
Joe Newell produces a cross, clearance by Jos Hooiveld.15:10
The assistant referee flags for offside against Tyrone Barnett. Indirect free kick taken by Kelvin Davis.14:34
Corner taken right-footed by Lee Tomlin to the near post, Jose Fonte manages to make a clearance.14:28
The ball is swung over by Lee Tomlin, clearance made by Jose Fonte.13:49
Lee Tomlin delivers the ball, Tyrone Barnett takes a shot. Save by Kelvin Davis.11:18
Adam Lallana takes a shot. Save by Paul Jones. Inswinging corner taken right-footed by Dean Hammond played to the near post, clearance made by Shaun Brisley.10:54
The ball is swung over by Danny Butterfield, Lee Frecklington makes a clearance.9:32
Assist by Richard Chaplow.9:32 Goal scored
Goal! - Billy Sharp - Peterboro 0 - 2 Southamp'n Billy Sharp grabs a goal from deep inside the penalty box to the bottom right corner of the goal. Peterborough 0-2 Southampton.9:31
Adam Lallana sends in a cross.8:47
Long range shot from Daniel Fox misses to the left of the target.8:13
Danny Butterfield delivers the ball, clearance made by Lee Tomlin. Adam Lallana takes the inswinging corner, clearance made by Tommy Rowe.6:30
Gabriel Zakuani challenges Richard Lambert unfairly and gives away a free kick. Danny Butterfield restarts play with the free kick.4:52
Daniel Fox provided the assist for the goal.4:52 Goal scored
Goal! - Jos Hooiveld - Peterboro 0 - 1 Southamp'n Jos Hooiveld grabs a headed goal from inside the six-yard box. Peterborough 0-1 Southampton.4:19
Inswinging corner taken by Daniel Fox from the right by-line,4:09
Centre by Billy Sharp, clearance made by Gabriel Zakuani.1:45
Tyrone Barnett fouled by Jose Fonte, the ref awards a free kick. Free kick taken by George Boyd.0:39
Danny Butterfield delivers the ball, clearance made by Shaun Brisley.0:00
The referee gets the game started.
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