Intrepid first signalled it had outgrown its five Fitzroy offices last year, putting out a requirement for space in the CBD or city fringe.
Since opening its flagship Melbourne store on Bourke Street in 2005, it has built local offices in Cambodia, China, Ecuador, India, Kenya, Morocco, Peru, Thailand and Vietnam.
Its purchase of British-based travel giant Guerba in 2006 has given Intrepid a piece of the lucrative British outbound market and inbound African market.
CB Richard Ellis and Colliers International are marketing 649 Victoria Street.
A spokesman for the leasing agents declined to speculate on a big mystery tenant, saying a formal deal had not been signed.
Intrepid Travel confirmed that the property was one of a handful it was reviewing for its 3000 sq m requirement.
A spokeswoman said it was interested in an environmentally friendly, modern building, and expected to make a decision within weeks.