A regime shift is an abrupt, substantial, and persistent change in the state of a system. We show that a regime shift in the September Arctic sea-ice extent (SIE) occurred in 2007. Before 2007, September SIE was declining approximately linearly. In September 2007, SIE had its largest year-to-year drop (by a wide margin) in the entire 46-year satellite record (1979-2024). Since 2007, September SIE has been approximately constant, i.e., no long-term trend. The regime shift in 2007 was caused by significant export and melt of older and thicker sea ice over the previous 2 to 3 years, as documented in other studies. We test alternatives to the traditional linear model of declining September SIE, and discuss possible explanations for the lack of a trend since 2007.