The 22-year-old French international is currently in Russia with his national team ahead of the World Cup and has previously been linked with Arsenal and Liverpool.
Lemar has had a disappointing season for the French club having helped them reach the semi-finals of the Champions League and win Ligue 1 in 2016-17, but is still highly rated and has scored 22 goals in 123 games for Monaco since signing from Caen in 2015.
This announcement has sparked much speculation regarding the transfer plans of both Atlético Madrid and Liverpool. Antoine Griezmann yesterday said he had decided on his future following rumours of a move to Barcelona but that it "wasn't the time or place to talk about it." Griezmann had previously encouraged Madrid to invest in their squad or risk losing him, so the signing of Lemar could be seen as a statement of intent.
Following the announcement of the preliminary agreement, Atlético said on their website:-
Atlético de Madrid and AS Monaco have achieved a preliminary agreement over the transfer of Thomas Lemar to our club. Over the next days both clubs will work to close the agreement for the definitive transfer.