Gov. Jack Markell and Attorney General Beau Biden planned to join lawmakers Thursday in Wilmington to announce the filing of the legislation.
Crafting of the bill has been led by members of the gay rights group Equality Delaware.
Markell has pledged to sign the bill if it passes the General Assembly, where it has the support of Democratic leaders in both chambers.
In 2011, Markell signed a bill authorizing same-sex civil unions in Delaware, effective last year.
Critics of the civil-union legislation warned at the time that it was simply a precursor to same-sex marriage in Delaware, which could soon join nine other states that have legalized gay marriage.