Marsh-Brown has joined League One side MK Dons in a one-month loan deal.
The England youth international has impressed for Fulham reserves this season.
Dons boss manager Karl Robinson told the club's official website: "Keanu played against us in a reserve game last week and was extremely impressive. We spoke to Ray Lewington (Fulham's academy technical director) and Billy McKinlay (reserve team manager) about him and they only had good things to say about the boy.
"He fits with the profile of player we've been looking for and the type of player we want to bring into this football club. He's young and hungry with great enthusiasm and pace and his versatility will be extremely valuable to us.
"It's another exciting young player our fans can look forward to seeing and hopefully Keanu can take a lot of positives from his loan spell with us."