The 20-year-old will remain with hometown club Botev Plovdiv, for whom he has scored 16 goals in 57 league appearances, until the new year before moving to the Coface Arena for an undisclosed fee.
Nedelev told Mainz's official website: "Moving to Mainz if the best step for me. I will play in one of the best leagues in the world and for a club who will support me and give me the time I need to become a good footballer.
"I know (I) have a few months' time to prepare well and for now I will give everything to Botev Plovdiv, to whom I am very grateful."
The left-footed winger currently tops the fledgling Bulgarian A Football Group scoring charts with five goals from six games and made his senior Bulgaria debut in a friendly game with Macedonia on August 15.
Mainz manager Christian Heidel said: "Todor Nedelev is an extremely prolific player and can be used in every attacking position. He is a very young player but he is amazing and already has a lot of experience."