The Boston Celtics have been a cornerstone of the NBA since 1949, but even with decades of dominance, the 2026 playoffs delivered a gut punch that will echo in franchise history. For the first time ever, the Celtics blew a 3-1 series lead, losing to the Philadelphia 76ers in the first round. This wasn't just a loss—it was the most painful first-round collapse in the team's storied past, and it's one that Celtics fans won't soon forget.

Historically, first-round exits as a higher seed have been rare for Boston. Before 2026, it had only happened five times: in 1951, 1952, 1956, 1990, and 1993. Those earlier losses, however, came as a No. 4 or No. 2 seed. The 2026 defeat was different. The Celtics entered as the No. 2 seed, while the Sixers were the No. 7 seed—making this the first time in the modern playoff era (since 1983-84) that Boston lost in the first round as a top-three seed.

The shock factor was amplified by expectations. According to bet365, the Celtics entered the postseason with the best odds in the Eastern Conference to win the 2026 championship at +550. The Sixers, meanwhile, were long shots at +17,500, ranking seventh in the conference alone. Boston was never seen as an underdog in this series—they were supposed to be contenders.

So, how does this compare to other painful playoff losses in Celtics history? The team has had 12 instances where they lost as the higher seed in the Eastern Conference Semifinals or Finals. Some of the most memorable include 2009, 2020, 2023, and 2025.

In 2009, the Celtics were defending champions but were without Kevin Garnett due to injury. As the No. 2 seed, they fell in seven games to the No. 3 seed Orlando Magic. In 2020, Boston, again the No. 2 seed, lost to the No. 5 seed Miami Heat in the Eastern Conference Finals—though Miami was on a historic run in the NBA Bubble, having gone 8-1 in the playoffs and upsetting the top-seeded Bucks.

While those losses stung, none carried the same weight as the 2026 collapse. This wasn't a series where the opponent was on a hot streak or the Celtics were missing a star. This was a fully healthy, favored Boston team that simply couldn't close the deal. For fans, it's a scar that will take a long time to heal—and a reminder that in the NBA, no lead is ever safe.