illegal immigrant alien in jail marry a U.S. citizen, prohibited marriages in WA state

If the parties are in WA and the marriage would not violate RCW 26.04.020 (Prohibited marriages), the parties likely can marry each other. The statutes can be found here: http://apps.leg.wa.gov/RCW/default.aspx?cite=26.04 .

Since one of the parties is in jail, special arrangements with the jail authorities will have to be made. Depending on why the person is in jail, the jail authorities may not be willing to help.

The marriage may not be helpful in resolving the alien's immigration problems, especially if the alien is in immigration holding awaiting deportation.

You may want to discuss your facts and options with an attorney.