The Ligue 1 club bought the 22-year-old from Marseille last June for £13million.
Mendy's performances have since attracted plenty of interest from the Premier League, including Manchester United, Liverpool and Manchester City.
The France international shone in the Champions League against City, and his valuation has quickly risen.
According to the Mirror, Chelsea will have to spend around double the price Monaco paid for Mendy less than 12 months ago.
Benjamin Mendy has attracted much interest for his performances at left-back for Monaco
Mendy would certainly fight for a spot at Stamford Bridge
Antonio Conte wants high-quality reinforcements ahead of the Blues' return to Europe's elite club competition.
Left-wing back Marcos Alonso cost Chelsea £23m last summer, and he has been a consistent figure in the team's Premier League success.
But the Spaniard has little competition for a place and Mendy would certainly fight for a spot at Stamford Bridge.
Chelsea transfer news: Five most likely summer deals Wed, March 29, 2017
Express Sport brings you the five most likely summer signings Chelsea could seal according to Betfair
Play slideshow Getty Images 1 of 5
Virgil van Dijk – 5/1
Chelsea could afford the potential figure worth around £26m to land Mendy, but their rivals are poised to fight for his signature.
Get Quotes on Home Insurance
City and Liverpool are both attracted to the rising star, as Pep Guardiola and Jurgen Klopp are each actively looking for a left-back at the end of the season.
Meanwhile, United boss Jose Mourinho is reportedly looking to raid Monaco for Mendy and two of his team-mates in Chelsea target Tiemoue Bakayoko and Djibril Sidibe.