My stepdaughter had a baby (now 2 years old) with two men being the possible father. The one she was with when she found out took full responsibility knowing this. He signed the birth certificate and another form stating he is the father and would never dispute that and they got married. They have split recently and she is with the other possible father and wants to get a DNA test hoping the one she is with now is the father and wants to take all rights away from the one who signed the birth certificate. My question is does the father who signed the birth certificate have custody rights if it is proven he is not the biological father?

What State? It depends on several factors.

First she should get the DNA testing done. Then she should worry about custody, visitation and support.

Have you read this? https://www.askmehelpdesk.com/family-law/putative-father-registry-344695.html

I am concerned with his rights. She did wrong in the relationship and I am hoping he can stay as the father to his daughter. The other man hasn't been in the picture till now. This is in Mississippi.

Since he signed for the child then he has all the rights and responsibilities that go with it. Also he couldn't have signed any paper stating he would never contest it because that paper would be declared illegal as he would be waving rights to representation without full knowlage of the law.

If a person acknowledges paternity, then they are assumed to be the legal father unless the bio father challenges it. The kicker is that the bio father may only have a limited time to fight it. This varies from state to state. I would suggest the presumed legal father consult a local Family law attorney to determine whether his paternity can now be challenged.

IThe kicker is that the bio father may only have a limited time to fight it. This varies from state to state.

No time limit in Mississippi.