Police have issued CCTV images of a man they want to speak to following the theft at Cheltenham Spa railway station on Thursday, November 1.
They have released the pictures for the first time today.
The dark grey Marin mountain bike was left secured in the cycle racks in the morning, but was gone in the evening.
PC Julian Ribchester said: "We have carried out several lines of enquiry, including speaking to witnesses and reviewing CCTV footage and have since identified a man we would like to speak to as we believe he will have information which can assist the investigation.
"If you know the man, or his whereabouts, then please get in touch."
Anyone with information should contact British Transport Police on 0800 40 50 40 quoting incident 547 of 01/11/2012.
