One men approached box office staff, threatened them and took cash from the till.
He also took a mobile phone and a set of keys. Staff were left uninjured but shaken.
One suspect is 5ft 8ins, about 40, slim, with grey hair and wearing a black blazer. The other is 6ft, slim, and younger than his accomplice. He was wearing a dark green jacket with the hood up.
The robbery took place just before 5pm on Tuesday evening.
Detective Constable Joe Keith said: "We are checking CCTV.
"The Trongate is usually busy at this time of day."