The 33-year-old defender joins the club having spent the 2021 season in the US with North Carolina Courage, and has had previous spells in Germany, Iceland, Norway and her native Ireland, throughout her career.
Caldwell is also an experienced international player with 84 caps for Ireland.
Diane Caldwell said:
“Today is one of the most special moments in my life so far. As a lifelong United fan, it’s a dream to sign for the club and it’s a big moment for me and my family. I’m so grateful to be here and will wear the badge with so much pride.