The Scottish Premiership side have had a busy transfer deadline day and do not look like they are finished just yet.
The 22-year-old, who has won seven caps for his country, has made 17 appearances for Fulham since joining from MK Dons. However he has spent much of his time out on loan.
Williams is MK Dons' youngest ever goal-scorer, yet his first strike for the Dons remains his only recorded goal in professional senior football.
Nevertheless, the forward remains a bright talent, highly rated by both his club and country, and Fulham manager Slavisa Jakonovic will be hopeful that Williams can return to Craven Cottage come the end of the season having found his scoring boots.
St Johnstone currently sit eighth in the Scottish Premiership table on 26 points, tied with seventh-placed Kilmarnock.
Williams could make his debut this weekend against Hearts away at Tynecastle, as his new club travel to Edinburgh in search of their eighth league win of the season.