The left-sided winger was released by the Hammers at the end of last season after working his way through their famed academy system over the last few years.
Diallo had made 25 appearances for West Ham’s under-21s and earned international honours when he featured alongside the likes of Chelsea midfielder Carney Chukwuemeka and Bayern Munich star Junior Musiala for England’s under-17s.
His search for a new club led to a trial at Chelsea and he spent time with Derby County in pre-season as they looked to rebuild following their relegation into League One.
The winger featured in a number of pre-season games for the Rams and earned praised from then-Rams interim manager Liam Rosenior following a promising display in a friendly defeat at Stevenage.
He told the Derby Telegraph: “He showed some nice touches and it was a difficult game to come into.
“He is somebody we have been aware of for a long time. Our recruitment team have been working really hard in the background and we felt it was a really good opportunity to see him in a game.
“He’s been on trial at Chelsea and has a really good pedigree. We just have to make sure we make the right additions at this moment. We’ll take it from there.”
However, he left the League One newcomers without earning a deal and Newcastle are his latest point of call.
The winger featured for the Magpies under-23s in their 1-1 draw with North East rivals Middlesbrough on Friday night, playing for the first 55 minutes before being replaced by Michael Ndiweni.
Diallo is represented by CAA Base, the same agency that works with United first-team players Kieran Trippier and Chris Wood, as well as youngsters Dan Langley, Dylan Stephenson and Jordan Hackett.