The two have linked up many times on international duty for France and have now joined forces at United after Varane made the switch from Real Madrid.
Centre-back Varane commanded a fee believed to be in the region of £42.7million (€50m) and Martial expects him to take to life at Old Trafford with the ease of a man who has won the World Cup and Champions League.
"Varane, I know already, of course," Martial, who is also excited by the earlier arrival of Jadon Sancho, told United's official website. "And the skill set he has, I know he will bring a lot to our team, in addition to his long experience, so we are all really delighted with these two new players we have.
"Varane is a big-time player. I don't think he will need too much time to adapt to his new environment, but I'm there for him for whatever little help or advice he may need.
"He has all his experience. He's a player who has won a lot during the time of his career at Real Madrid and also in the French national team, so I am sure he will bring all his powers, as he really is a top player and I know things are going to go well for him here at United."
Having moved to the Santiago Bernabeu in 2011, Varane has won three LaLiga titles and the Champions League on four occasions.
He racked up over 350 appearances for Los Blancos and arrives at Old Trafford as one of the world's finest defenders.
Indeed, Varane has played 79 times for France, winning the World Cup in 2018 before featuring in all four of Les Bleus' games at Euro 2020.
Ole Gunnar Solskjaer's side opened its Premier League season with an emphatic 5-1 win over Leeds United.
Varane could make his debut in Sunday's trip to Southampton.