Part of Middlesbrough's academy and development squad for more than a decade, the 19-year-old is described as a ‘tall and athletic midfielder.’
He becomes the third signing of the January transfer window, with more expected ahead of the 11pm deadline on Monday.
Fletcher follows Jake Hull on-loan from Rotherham United and Marcus Carver for an undisclosed fee from Southport.
Reacting to the signing, Graeme Lee said: “We have been in talks with Boro for a while so I am delighted to get Isaac over the line.
"He is a real attacking threat and can play in several positions.
"He has showed over the last few weeks that he can create and score and I expect him to make a big impact on our side.”
Fletcher added: “I am delighted to sign on loan for Pools. I obviously know the Gaffer after working under him at Boro.
"I know what he expects from his teams, he has high standards and this a great opportunity for me.
"I have set myself targets in both goals and appearances and I want to give my all for this team.”
The 6ft 4” attacker already has family connections at Pools – his younger brother, Oscar, plays for the Pools Youth side.
Fletcher will wear the number 21 shirt for Pools.
Subject to FA clearance he is expected to be in the squad for the trip to Exeter on Saturday.
On Tuesday night, Lee admitted he was still keen to strengthen in the attacking areas on the field as well as bringing in a goalkeeper to compete with penalty hero Ben Killip.
Pools have had players on trial in recent weeks while they weigh up their options as well as making contact with both Manchester United and Arsenal over potential loan deals for some of their academy players as was revealed by chairman Raj Singh at the weekend.
Meanwhile, Hartlepool United will discover their Papa John’s Trophy semi-final opponent on Saturday teatime.
Graeme Lee’s side overcame Charlton Athletic on penalties to book their spot in the semi-finals of the Papa John’s Trophy.
The EFL has confirmed the semi-final draw will take place live on Sky Sports on Saturday 29 January following the conclusion of Peterborough United’s Sky Bet Championship fixture against Sheffield United (5:30pm kick-off).