Martha spent the second half of last season on loan at Liverpool, and prior to her move to Spurs, spent two seasons with Manchester United.
After going through the US collegiate system with Charlotte 49ers, the 30-year-old joined Le Havre, before moving to England to play for West Ham United, scoring 10 goals in 33 games.
Upon signing, Thomas said: “I'm really excited, it’s a really big club to be able to play for and I just can't wait to get started. I want to compete and play for the best teams.
“Team culture is really important to me and I think I bring some good attributes on that to help the team in the dressing room and push standards on the pitch.”