A transmission device for a bicycle includes a housing, a mandrel, an assembling member, a transmission assembly, a derailleur assembly and a clutch assembly. The mandrel is rotatably disposed on the housing and extends along an axial direction. The assembling member has a driving gear. The transmission assembly is rotatably disposed on the housing and engaged with the driving gear. The derailleur assembly includes a first derailleur gear and a second derailleur gear. The transmission assembly is engaged with the first derailleur gear and the second derailleur gear. The clutch assembly includes a clutch member.