"Paris St Germain is the greatest club in France," Herrera told PSG's website. "It keeps on improving and competes to win every possible title.
"I was looking forward to coming to Paris and to wearing those colours ... Joining this club is an amazing feeling.
"Paris is the most beautiful city in the world. I promise three things: work, professionalism and passion."
Herrera, who came through the ranks at Real Zaragoza before joining Athletic Bilbao, made 189 appearances for United in all competitions and scored 20 goals. He won the FA Cup, League Cup and Europa League in his time at the English club.
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(Reporting by Simon Jennings in Bengaluru; Editing by Alison Williams)
