For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
If one doesn't believe in a Creator God then they can not claim to be Christian.
Colossians 1:16 NLT
 for through him God created everything in the heavenly realms and on earth. He made the things we can see and the things we can't see- such as thrones, kingdoms, rulers, and authorities in the unseen world. Everything was created through him and for him.
I find it interesting that the questioner asked if Christians have to believe in "Creationism" and not just do they have to believe in "Creation".
Christianity is the belief in a Creator God who made man, and had a relationship with them, against Whom man rebelled, Who then sent His Son to pay the penalty for man's rebellion and sin, in order to restore a relationship between God and man that was lost when man rebelled, and that faith (belief and trust) in recognizing and accepting that only the finished and complete work of Christ can save us from the just judgment for our sins and reconcile us to God, who then receives us as His children and gives us the promise that the restored relationship will last forever.
If God did not create, then man had no relationship with Him. With no relationship, we couldn't rebel and lose that relationship. So, then Christ would not have had to come to die to restore that non-existent relationship...and the very premise of "Christianity" is gone.
So, yes. A Christian must believe in God as the Creator: thus they must believe in Creation.
Creationism is a branch of forensic science that seeks to show that the data we have uncovered in physics, chemistry, biology, geology, etc. is BEST explained within the context of the events of Genesis: creation, flood, language, the origins of ethnicity/race. It is not necessary to believe in one or another statement or prior or current conclusions of creationism to be a Christian.