Figures show Glasgow City Council made £2,469,420 from fines issued to drivers wrongly using the city's 16 camera-enforced bus lanes last year.
The local authority has previously said the money goes towards administering and collecting fines.
But an investigation found the cost of sending out and collecting penalties totalled just £437,453 - leaving a profit of more than £2m (£2,031,967).
A spokesman said the surplus is invested in local transport strategies.
Figures released under the Freedom of Information Act show the most profitable bus lane camera is on Glassford Street, where 23,250 motorists were caught in 2013, netting £545,280 for the council.
There are a total of 173 bus lanes of varying lengths across Glasgow but, of these, only 15 are regulated by cameras that produce automatic fines for cars caught in them.
The £60 fine is reduced to £30 if paid within 14 days.
A 16th camera, on Cathedral Street, was removed in March last year after a section of the road was closed to traffic.
A spokesman for Glasgow City Council said: "Money from bus lane penalty charge notices is invested in transport strategies.
"These are a balanced strategy concentrating on promoting and enhancing sustainable transport options.
"These include walking, cycling, public transport, investment in roads through tackling key congestion points, essential links to development areas and links to enable public transport to provide effective services."