The 25-year-old defender has made 12 appearances in Portugal this season and his performances have impressed Monaco coach Claudio Ranieri.
Echiejile has 38 caps for Nigeria and was part of the side that won the Africa Cup of Nations in 2013 and qualified for this year's World Cup. He headed to Portugal from Rennes in 2010 and becomes the second arrival this month at Ligue 1 club Monaco after Lacina Traore joined from Anzhi Makhachkala.
"I am very happy and proud to return to France as a club AS Monaco FC is one of the best in Europe," he told the club's official website. "(They have) a great reputation in Africa since George Weah was found there. In addition, great Nigerian players wore this jersey like Lukman Haruna (and) Victor Ikpeba and I'll give the best of myself to succeed them as best possible.
"I've got to know some of the history of the club, I have heard great things about the club and I am very happy to be part of the project. It's a great thing happening to me. Being in a team with really good players is good, you get to improve individually. I just want to be here, achieve a lot and win trophies."
Echiejile could be in line to make his debut against Toulouse on Sunday, a game Ranieri believes is vital if his side is to get its Ligue 1 title charge back on track.
"Everyone is very concentrated on Toulouse and works well together," said Ranieri in a press conference. "It will be a very difficult match and we must win to pick up the pace. Toulouse are a well organized team, cover the spaces well and play very quickly. This game is an opportunity for us to recover the points we lost recently. This is the time to show our determination. "