The deal is reportedly worth an initial £51.8m, marking a significant coup for new manager Thomas Frank.
Spurs 'hijacked' the transfer after Chelsea, who had tracked Simons all summer, failed to offload out-of-favour players.
Simons, a product of Barcelona's La Masia academy, has signed a long-term contract and can play across various attacking midfield roles.
This signing addresses Frank's need for an attacking midfielder, especially following James Maddison's long-term injury and previous failed pursuits of other targets.