Charing Cross (Bakerloo and Northern lines) is the closest Tube station, with an entrance/exit onto Trafalgar Square.
The following stations are within a few minutes walk:
Leicester Square (Northern and Piccadilly lines)
Piccadilly Circus (Bakerloo and Piccadilly lines)
Embankment (Bakerloo, Northern, District and Circle lines)
Charing Cross mainline rail station is about a three minute walk from the Square.
Routes 3, 6, 9, 11, 12, 13, 15, 23, 24, 53, 77A, 88, 91, 139, 159, 176, 453 go past Trafalgar Square. Also routes 22 and 94 terminate nearby at Piccadilly Circus. Many of London's night buses serve Trafalgar Square. The following routes start/end near the square: 53, N2, N5, N18, N20, N21, N26, N29, N41, N47, N50, N89, N91, N97, N279, and N381, and these routes go past the square: 6, 12, 23, 24, 88, 139, 176, 453, N3, N9, N11, N13, N15, N36, N44, N52, N77, N155, N159, N343.