The Estoril Open, officially known as the Millennium Estoril Open for sponsorship reasons and formerly also known as the Portugal Open, is a men's professional tennis tournament. It is part of the ATP Tour 250 category and takes place annually on the clay courts of the Clube de Ténis do Estoril in Estoril, Portugal. Founded in 1989, until 2014 the women's tournaments were also played and the competitions were held in Oeiras.

It was inaugurated in 1989 as a women's tournament in the WTA Tier V category with the name Estoril Open on the clay courts of the Complexo Desportivo do Jamor in Oeiras, a sports complex that also includes the Jamor National Stadium.

In 2013 the tournament was renamed the Portugal Open, and continued to be played in Oeiras until 2014, when the women's tournaments were played for the last time. In 2015 the tournament took the name of the sponsor and was called the Millennium Estoril Open, since then only men's tournaments have been held and take place at the Clube de Ténis do Estoril in nearby Estoril. The 2020 edition was not held due to the COVID-19 pandemic.