The Belgium boss has jumped ahead of Andre Villas-Boas and Neil Lennon in the market after Paddy Power were forced to slash odds following a flurry of bets.
Martinez went from 4/1 to 4/6 despite being contracted to the Belgian FA until after Euro 2020.
A spokesperson for Paddy Power said: “Martinez’s Belgium are amongst the favourites to win Euro 2020 next summer, so this would be a strange one.
“Then again, maybe he wants more of a guarantee of silverware, which he’d certainly get at Celtic, as they close in on nine consecutive Scottish trophies.”
Martinez, a former Motherwell player, started his managerial career at Swansea City who he led to the League One title in England before moving on to Wigan Athletics. He won the FA Cup with the Latics but oversaw their relegation from the Premier League in the same season.
He had mixed success at Everton before being appointed Belgium boss and took them to the semi finals of the World Cup in Russia last year.