However, the France international's 81st minute strike would only have brought one point but for a fantastic late save by David James.
Bolton complained bitterly that the ex-Chelsea and Arsenal midfielder's goal was offside and perhaps with good justification.
Substitute Kanu had the final touch after the ball deflected to him from Portsmouth old boy Andy O'Brien and Diarra finished beyond Jussi Jaaskelainen.
Wanderers knew their luck was definitely out when James flung himself to deflect a stoppage-time effort from substitute Tamir Cohen over the bar.
The England man was in inspired form, justifying Fabio Capello's decision to bring him back from the international wilderness.
He denied Bolton man-of-the-match Kevin Davies, plus Gretar Steinsson and Grzegorz Rasiak while Hermann Hreidarsson cleared off the line from Gary Cahill.
Portsmouth had rarely threatened before Diarra's winner, though the same player tested Jaaskelainen from distance and Kevin Nolan did well to block a goal bound Papa Bouba Diop effort.
However, for all James' brilliance he should have been picking the ball out of the net in the opening period.
Matty Taylor, who missed a penalty at Reading the previous week, should have opened his Bolton account.
Only Taylor will know how he didn't have Wanderers in front by the break.
The former Pompey midfielder already managed a couple of sighters before his miss of the season contender.
Indeed, he didn't really require any rehearsals as initially El Hadji Diouf and then Davies served up the easiest of chances.
But three yards out from an empty net, he contrived to blaze the ball over the crossbar and high into the stands.
Bolton certainly deserved a goal for their first half display as, even in the shock 1-0 home win against Manchester United, they hadn't played with the same fluency or skill level.
Portsmouth didn't manage a shot at Jaaskelainen in the opening half.
And they also lost Noe Pamarot with a knee injury following Danny Guthrie's robust but legal challenge.
Bolton fans wondered whether their side had missed the boat, especially when Taylor fluffed another inviting opening in the second period, and so it proved as Diarra punished Bolton's profligacy.