Hammers looked to have won the game when Carl Fletcher curled home a superb strike in the 87th minute of a hard-fought encounter at Upton Park, but the visitors responded just moments later with a close-range equaliser from striker Vincent Pericard.
Coach Kevin Keen fielded a strong side, with no fewer than eight players possessing first-team experience. Goalkeeper Shaka Hislop added some welcome presence between the sticks, while Welsh defender James Collins was tried out in the right-back role and Israeli striker Yaniv Katan played after making his Premiership debut as a substitute at Aston Villa on Saturday.
The hosts had a lucky escape after just five minutes, when Collins turned the ball into his own net following a low cross from former Hammer Svetoslav Todorov, who was thankfully flagged offside.
However, that was the only real action of any real note until the half hour mark, when Portsmouth escaped on the counter-attack and Hislop had to be alert to make a smart save from the dangerous Franck Songo'o.
Five minutes later, Hammers produced their first effort of the night, when Katan was fouled 25 yards out and Collins stepped up to curl in a free-kick that Pompey keeper Guatelli safely palmed over the bar.
Clive Clarke then saw a similar effort fly wide of the target, and both sides headed off at the interval without having seriously tested the opposition goalkeeper.
The second half followed much the same pattern, although Hammers' experience began to tell as the game wore on and, with Fletcher and Mark Noble pulling the strings in midfield, the hosts enjoyed the lion's share of possession.
Fletcher went close with a couple of fierce long-range efforts that flew just wide of the target, while Jeremie Aliadiere missed the best chance of the match on the hour mark when he jinked between two defenders in the penalty area and flicked the ball just inches wide of the post.
On 64 minutes, the impressive Katan came off to be replaced by Czech striker Petr Mikolanda, who scored Hammers' goal in last week's 1-1 draw at Southampton, while young left-winger Kyel Reid came on for his Youth Academy colleague Hogan Ephraim soon after.
Fletcher again hinted at what was to come with a measured half-volley from Noble's clever pass into the penalty area, but Guatelli was equal to the effort, and as the game drew to a close, it seemed that the small Upton Park crowd would have to settle for a 0-0 draw.
However, with just four minutes remaining, the game burst into life. A rare Portsmouth attack ended with the ball dropping eight yards out at the feet of Todorov, whose point-blank effort was somehow kept out by a stunning save from Hislop.
From Shaka's resulting clearance, the ball was helped on to Mikolanda, whose neat lay-off found Fletcher. The midfielder advanced to the edge of the area before curling a fantastic effort past Guatelli and into the corner of the net.
It appeared that the goal would give Hammers all three points but, just a minute later, Pompey surged forward and after a fierce drive from Songo'o had cannoned back off the post, Pericard was on hand to stab the ball home from six yards out.
Hislop was called into action again in stoppage time to deny Songo'o, but Hammers held on for the point and are now unbeaten in their last four matches.
Kevin Keen said afterwards: "It was a very close game and, in the end, I would say a draw was probably a fair result.
"It was a case of two experienced sides, bolstered with a few youngsters, and for 87 minutes it was a bit of a stalemate, with both defences on top and few chances at either end.
"I was pleased with the players, though, and thought they put in a lot of effort. Fletch scored a super goal and I thought we had it in the bag with a couple of minutes to go, but they had some experienced guys up front and caught us napping moments later.
"It was a good work-out for us and there were again some positives to take from the game."
Hammers: Hislop; Collins, Tomkins, Blewitt, Clarke (Andrews 81); Ephraim (Reid 75), Baradji, Fletcher, Noble; Katan (Mikolanda 64), Aliadiere. Unused subs: Kemal, Hines, Widdowson.