Wigan notched a first home win of the season with a typically slick passing display against a sloppy West Ham.
Ivan Ramis gave the Latics an early lead with a magnificent volley straight from Jean Beausejour's corner.
James McArthur grabbed the hosts' deserved second with a 15-yard strike after Jussi Jaaskelainen's poor kick.
West Ham could have gone fourth with a win, but they failed to trouble Wigan keeper Ali Al-Habsi until James Tomkins headed a late consolation.
The Hammers began the day 10 places above their hosts in the table, but the outcome rarely looked in doubt after Ramis's brilliant eighth-minute strike.
Conor McNamaraBBC Match of the Day commentator
"Look out for Ivan Ramis' volley tonight - he showed the kind of technique normally reserved for the likes of Paul Scholes. Stuart Hall said on Radio 5 live that it was the best goal he had ever seen!"
Roberto Martinez's Wigan side, without a league win since August, grew in confidence after the goal and their rhythm was rarely interrupted by the visitors.
The Hammers repeatedly gifted possession to the home side as their promising early-season form deserted them.
This had been billed as a match of contrasting styles - Martinez's elegant passing game versus Sam Allardyce's percentage football - so Wigan seemed to be going against type when they scored their opener from a set piece. It was, however, the most spectacular of set pieces.
Beausejour sent in a corner and Ramis was allowed to drift unmarked to a position 14 yards out, from where he unleashed an unstoppable left-footed strike past Jaaskelainen.
Ramis strike delights Martinez
The Spanish defender, who had agreed terms with West Ham before opting to join Wigan from Mallorca in the summer, also hit a wonder strike against the Hammers in a 4-1 League Cup win last month.
The game then settled into its predicted pattern: the Latics knocked the ball around sweetly while the visitors pumped it forward in search of Andy Carroll's head.
The home side's tactic looked more effective, with James McCarthy, Shaun Maloney and Beausejour all going close.
Carroll got a first sight of goal just before half-time when he headed Tomkins' cross wide, but the Hammers' hopes of mounting a second-half fightback were snuffed out within two minutes of the restart.
A woeful clearance from Jaaskelainen gifted Wigan possession and they took full advantage. Maloney helped Beausejour's pass on to McArthur, who slammed home from the edge of the box to double the lead.
The Latics were happy to soak up West Ham pressure for the remainder of the match, but the Hammers toiled without reward and Carroll cut a frustrated figure.
West Ham were lacklustre - Allardyce
When his opportunity finally arrived from an accurate Matt Jarvis cross midway through the second half, the England striker's control let him down.
In fact, Allardyce's side failed to muster a clear attempt on goal until the final minutes when Tomkins's close-range volley hit the bar.
The young defender did get on the scoresheet moments later with a simple header from George McCartney's cross, but there was little time to push for what would have been a scarcely deserved leveller.
There was a sense of relief in the Wigan ranks at the final whistle as the Latics celebrated a belated return to the form they showed at the tail-end of last season.
A win would have sent West Ham into a Champions League position, but with games against Manchester City, Manchester United, Tottenham and Chelsea in the coming weeks, this defeat could provide a reality check on their ambitions.
Wigan manager Roberto Martinez:
"We don't get too many goals from set plays, but it was a tremendous finish from Ivan Ramis.
"That early goal set the tone of the game and allowed us to control it.
"I really enjoyed the performance. We have been showing flashes of our ability but today we were really good.
"Our philosophy and way of playing will never change."
West Ham manager Sam Allardyce:
"It was a bit of a push on Winston Reid for their first goal, but it was a brilliant volley. And they deserved to win.
"We didn't play any football today, we didn't get past half-decent. I'm disappointed our performance has reduced to such a level.
"It wasn't about Wigan today, it was about us not using the ability we've got. We can't keep giving the ball back to them."
"We were not good enough, in any shape or form. Our problems were all over the pitch."
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