Yes, lemons are generally considered tropical plants, although they thrive in subtropical regions as well.
While citrus fruits, including lemons, are grown worldwide, their origin and optimal growing conditions place them firmly within the tropical plant category. Most commercial lemon groves are found in subtropical areas because these areas provide a suitable balance of warmth and moisture for optimal growth and fruit production. This allows for larger harvests without as many of the common issues that some tropical regions can present.