Whereabouts in Trastevere are you? You would have to take the 23 on the other side of the river as Lungotevere on the Trastevere side of the river is one way going south. If you are anywhere near Ponte Garibaldi, you could walk (or take the No 8 tram) to Largo Argentina and pick up the 492 outside the Fetrinelli bookshop.
When we stayed in Trastvere we crossed the river using Ponte palatinio and took bus 280, and then walked the rest of the way
If you're closest to the Ponte Sisto, take the No. 23. There's a stop on the Lungotevere just before Ponte Sisto/Via dei Pettinari.
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