The contract is a two-way deal that in the first year would pay Allen $750,000 in the NHL and $125,000 in the AHL. The second year is a one-way deal that would pay Allen $850,000.
Allen made his NHL debut last season, going 9-4-0 with a 2.46 goals-against average and .905 save percentage in 15 games. He topped NHL rookie goalies in wins and GAA, and was second in save percentage.
"It's a situation where we still have [Elliott and Halak] up there on one-way deals, so it's a bit of a different situation for me," Allen told the St. Louis Post-Dispatch. "Obviously you want to be there, but you have to keep things in perspective and bide your time.
"I had a good season last year, but that's just a step. I think I still have to prove myself in camp. I'm looking forward to coming in and it's going to be a challenge. It doesn't matter about my contract ... I'm locked up for a couple of years."
A 2008 second-round pick (No. 34), he also played 35 games last season for the Blues' AHL affiliate, the Peoria Rivermen, going 13-19-2 with a 2.89 GAA, .904 save percentage and two shutouts.