Nikita joins from fellow Barclays Women’s Super League side Arsenal, after previous spells with Everton, Manchester City and Olympique Lyonnais.
The forward arrives in Manchester with an impressive list of domestic and international honours, including the Women’s Super League Cup, Women’s Super League, Women’s FA Cup, SheBelieves Cup, Arnold Clark Cup and the UEFA Women's Champions League.
The 28-year-old has 67 international caps and featured for the Lionesses at this summer’s UEFA Women’s European Championship, as part of the team that were crowned European champions following victory over Germany at Wembley.
On signing, Nikita said: “It feels unbelievable to be here.