The Swansea City player has been ruled out since September with a hamstring injury and has been training alone in Qatar since the squad arrived in the country last week.
However the 32-year-old joined the rest of the team in a training session on Wednesday and gives the team a boost ahead of their crunch Group B match in two days' time.
Allen was ruled out of Monday’s 1-1 draw against USA, which saw Gareth Bale rescue a point for the Red Dragons after falling a goal behind at half-time.
Wales must get a result against Iran if they harbour any hopes of qualifying for the latter stages of the competition for the first time in 64 years, as they face England in their final group match next week.
"I'd say this is a must-win game now after starting off with a solid point," midfielder Harry Wilson told reporters on Wednesday.
"Now we've got that point on the board, we're going into Friday's game to get the win. Hopefully we do that and then focus on the England game."
