The 50-year-old Argentinian gave his first press conference on Friday after signing a two-year contract earlier this week, and he was quick to shoot down talk of the Spaniard leaving.
"Manchester United want Cesc? If the club rejected two offers, I will reject it a third time. Cesc will be here," he insisted.
"It's normal United are interested, it's an honour, but we count on Cesc and we won't sell, no matter the offer."
When probed about other possible transfers for the Liga champions, Martino insisted he would need time before coming to any decisions - with his first focus on Barca's pre-season clash with Norwegian club Valerenga on Saturday.
"Signing a centre-back? I need more time to have an opinion, I didn't even see the squad train," he added.
"I will today travel to Oslo to join the team, I'll attend the game (against Valerenga) and then we return to welcome the Spanish and Brazilian players."
