The midfielder has been linked with a move to Tottenham and Manchester City this summer.
In quotes published by Sky Sports, the Everton boss responded bluntly when asked about McCarthy’s future, simply stating: "He’s not for sale.
"We are not in a situation at this football club that we have to sell to balance the books… We want to create a winning team.”
The Ireland international has been a stalwart at the club since his £13 million arrival two seasons ago and Martinez was quick to heap praise on one of his most important players.
“James McCarthy has been a top performer in the last few seasons. At the age of 24, to be over 300 games shows the know-how he brings, together with young legs and good energy.
"He is a vital part of our squad… It's important that we keep improving, keep getting stronger and that is the only focus we have - not losing assets."
That has left a McCarthy shaped hole in the middle of the pitch as Mauricio Pochettino seeks to find an upgrade on Ryan Mason to partner the impressive Nabil Bentaleb.
City’s interest comes after failure to secure a deal for Fabian Delph and the Everton man would add much needed bite and drive to an ageing midfield.
£20 million has been the reported valuation in recent weeks but given the apparent determination from the Everton gaffer, it make take a fair bit more than that to force a deal through.