The French striker has been struggling with an ankle injury in recent weeks.
But he came off the bench to help PSG beat Atalanta in the last eight and has trained since that game last Wednesday.
"He played for 30 minutes against Atalanta and had no reaction and had six days since to work on fitness, so of course he can start," said Tuchel.
"We will decide after training whether he can play 90 minutes."
Mbappe, 21, came on last against Atalanta with his team trailing 1-0 to the Italian side but inspired PSG to a late victory, setting up the winner for Eric Maxim Choupo-Moting in injury time after Marquinhos had levelled in the 90th minute.
He has scored 30 in 35 appearances in all competitions this season, including five in eight in the Champions League.
'Neymar is a different leader'
Mbappe's return to fitness will restore his potent attacking partnership with Brazilian forward Neymar, who has 19 goals in 25 starts this campaign and has impressed his manager with his leadership qualities.
"Since I arrived he has always been a leader. A different leader, not as everyone understands him, but with his qualities, his confidence, his courage in the field, his will to win," said German Tuchel. "He loves competition and you need that to be a leader.
"We also created a group with players like Ander Herrera, Pablo Sarabia, Keylor Navas and others, guys who know very well how to live together, created a collective atmosphere. Perhaps this is the key to seeing Neymar on another level."
As well as Mbappe, Tuchel will also have Argentine winger Angel di Maria available following suspension.
However, goalkeeper Keylor Navas, previously a Champions League winner with Real Madrid, is out with a thigh injury after coming off hurt late in the quarter-final, meaning Sergio Rico is likely to start.
In addition, midfielder Idrissa Gana Gueye is a major doubt, according to the club, because of a muscle injury.
For PSG, Tuesday's game represents a first European Cup/Champions League semi-final since 1995, having reached the last 16 for the previous three seasons and the quarter-finals four campaigns on the bounce before that.
"We are living in the moment. We deserve to be here because we have had an extraordinary season," said Tuchel, whose team won a domestic treble in France.
"We are absolutely ready, the hunger is there, but this is not a historic moment, it is a semi-final and we are here to make the most of all the hard work we have put in and to win it."
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