The defender returns to the WSL, following a previous spell in England with Arsenal, where she won the league title, League Cup and FA Cup.
Janssen’s most recent spell with German side VfL Wolfsburg saw her win the Frauen Bundesliga twice and the DFB-Pokal three times.
On the international stage, Dominique and her Dutch side were crowned European champions in 2017, following a 4-2 victory over Denmark in the final.
Dominique Janssen:
“I’m delighted to be at such a big club like Manchester United. Signing here is a really proud moment for me, and I cannot wait to now return to the English league and create new winning moments with this talented Manchester United team.