The distance from the Colosseum to Saint Peter’s square in the Vatican is 3.6 kilometers. You can get from one point to another by taking bus N64 (bus stop near Piazza Venezia and on the right side from St Peter’s Square (4 minutes by foot, name of the bus stop is Cavalleggeri/Fornaci). Another option is to use the metro, from the Colosseo metro stop to Termini metro stop (5 minutes), then from Termini metro stop to Ottaviano metro stop, located nearby the entrance to the Vatican Museums and finally, you have to go 10 minutes by foot to reach St Peter’s square and basilica.

Overall, it will take 30 minutes to get from one site to another