Vassilis Spanoulis (also commonly known as Bill Spanoulis or Billy Spanoulis, Greek: Βασίλης Σπανούλης; born 7 August 1982) is a Greek former professional basketball player and current professional basketball coach for Peristeri Athens of the Greek Basket League and the FIBA Champions League. Spanoulis spent the majority of his playing career in the Greek Basket League, winning four Greek League titles as a member of Panathinaikos Athens, and three Greek titles as a member of Olympiacos Piraeus. He also won three EuroLeague titles, one with the former club (2009), and two consecutive (2012 and 2013) with the latter. HoopsHype named Spanoulis one of the 75 Greatest International Players Ever in 2021. In 2022, he was named a EuroLeague Legend. He was also inducted into the Greek Basket League Hall of Fame in 2022.

During his pro playing career, at a height of 6 ft 4 in (1.93 m) tall, and a weight of 212 lb (96 kg), Spanoulis played as a point guard-shooting guard, and was nicknamed Kill Bill, V-Span, Greek Lightning, and MVP ("Most Vassilis Player"), during his playing days Spanoulis was named the Balkan Athlete of the Year in 2009, the All-Europe Player of the Year in 2012 and 2013, the Vatican's World Athlete of the Year (Giuseppe Sciacca Award) in 2013, and the EuroLeague MVP the same year. He earned a record eight All-EuroLeague Team selections and was voted the EuroLeague's 2010–2020 Player of the Decade. He is also the EuroLeague's all-time career leader in total points scored.

Spanoulis first played pro club basketball with Gymnastikos S. Larissas, and enjoyed a highly successful career start. His skill-set earned him a transfer to Athens and Maroussi, where he won the Greek Basket League's Best Young Player award for the 2002–03 season. Following an impressive 2004–05 season, during which he helped lead Maroussi to the Greek Basket League's finals and the 2005 EuroCup quarterfinals, he moved to Panathinaikos, where he became one of European basketball's major stars. In the 2005–06 EuroLeague season, Spanoulis made his debut in impressive fashion, earning his first All-EuroLeague Team selection as a rookie in the competition. After a stint in the NBA with the Houston Rockets during the 2006–07 season, he returned to Panathinaikos and helped lead them to a EuroLeague title in 2009, being voted the EuroLeague Final Four MVP in the process.

Spanoulis' transfer to Olympiacos from Panathinaikos in the summer of 2010 marked a new step in his career, given Olympiacos' feisty rivalry with Panathinaikos. Within a young and rebuilding Olympiacos team, Spanoulis not only led the club to a EuroLeague title run in 2012, but he also went on to lead the team to a historical EuroLeague championship repeat in 2013. In the process, he gained another two EuroLeague Final Four MVP awards, thus joining Toni Kukoč as the only players in the history of European basketball to achieve that distinction on three occasions. Under his leadership, Olympiacos reached another two EuroLeague Finals, in 2015 and 2017.

In 2017, European basketball historian Vladimir Stanković, named Spanoulis the "Most Clutch Player in EuroLeague History". In 2020, Spanoulis was named the "Best EuroLeague Player of The 2010s Decade" by the readers of the website Eurohoops.net.

Spanoulis played an instrumental role on the gold medal win of the Greece men's national basketball team at 2005 EuroBasket. He was one of the main stars of Greece's 2006 FIBA World Cup silver medal team, scoring 22 points in the memorable victory over Team USA (101–95), in the tournament's semifinals. It was however, Greece's bronze medal at the 2009 EuroBasket, that emphasized Spanoulis' leadership within an injury-plagued Greece national team, and consequently earned him an All-EuroBasket Team honor. In 2020, Spanoulis was voted one of the 25 Best Players in Summer Olympics history, since the inclusion of NBA players in 1992, and one of the 5 Best Players of the FIBA EuroBasket, since the year 2000.