The prospect has signed a three-year deal with the Hammers, as reported by Ajax Showtime.
Usually deployed on the left flank, El Mhassani is a skillful player and featured for Ajax at U16 level last season, although he missed several games through injury.
West Ham are renowned for their academy but at present supporters are yearning for senior signings with Pablo Zabaleta, who joined on a free transfer after leaving Manchester City, the only major arrival at the London Stadium so far this summer.
The Hammers finished in eleventh place in the Premier League last season but, unless they strengthen their squad significantly during the remainder of the transfer window, securing a top ten finish this time around could prove to be very difficult indeed.
Slaven Bilic’s team get their Premier League campaign underway when they face Manchester United away next month.