Jermaine McGlashan scored in stoppage-time to rescue a point for Cheltenham and deny Portsmouth their first away win of the season.
Patrick Agyemang had given Portsmouth the lead from close range, in their first competitive game with the Robins, after a stumble by Steve Elliott.
McGlashan levelled from Terry Gornell's pass but Johnny Ertl restored Pompey's lead from a cross by Jed Wallace.
McGlashan beat keeper John Sullivan to the ball to grab the late equaliser.
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Cheltenham Town manager Mark Yates told BBC Radio Gloucestershire:
"We've got to cut out individual and defensive mistakes. We're making bad decisions that are costing us goals and that's everybody.
"Jermaine did some really great things but like all of our players, he's not on top of his game. Even though he scored two goals, he's got to do things better.
"I've told them well done for showing some spirit and just go and think about your performance as an individual and as a team."
Portsmouth manager Guy Whittingham told BBC Radio Solent:
"I'd like to think our first-half performance was decent enough. Second half it was backs to the wall; we didn't keep the ball well enough, didn't move the ball well enough and ultimately all the pressure put on us cost us.
"The first half we created a number of chances, good chances again, and even second half we started brightly and then there seems to be something at the moment where we don't keep playing the football that we're capable of doing, so that we've got to look at.
"Look at the games we've conceded - Mansfield and here - of late. We've defended really well and coped with everything and Sullivan hasn't had much to do.
"It is frustrating, of course it is, but you know it's a good bunch of guys and we know we'll get this right."