The 20-year-old defender arrives at Huish Park for his first taste of EFL action after racking up the accolades at Stamford Bridge.
A Blue since the Under-8s age group, Grant was named Scholar of the Year in 2015/16 having already featured in a FA Youth Cup and Under-19s European triumph.
Rewarded with international recognition by making his England Under-18s debut in June 2016, the versatile defender – who can play all across the back four – won the treble in 2016/17.
Making 30 appearances for the Under-19s and Development Squad in 2017/18, the Londoner also featured against the Glovers in one of four Checkatrade Trophy appearances.
Joining up with the Town squad after Chelsea Under-21s’ Checkatrade Trophy clash with Peterborough United on Wednesday night, the youngster will go straight into the squad for Saturday’s trip to Mansfield Town.
Securing his first signature of the transfer window, manager Darren Way told YTFC.net: “Josh is powerful, athletic and has a good personality.
“This is a great opportunity for him to get regular first-team football and I’m pleased to have brought him in.”