Marine heatwaves (MHWs) have increased significantly over the past century, with global average frequency and duration rising by 34% and 17%, respectively, leading to a 54% increase in annual MHW days. This trend is largely explained by rising mean ocean temperatures. The study used satellite and in situ data to analyze MHW characteristics from 1925 to 2016, revealing significant centennial-scale increases in MHW properties and variability on interannual to multidecadal time scales. The satellite period (1982–2016) accounts for a larger portion of this increase than in situ records. The El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) each contribute to regional and global variations in MHWs. The study also found that changes in mean SST alone can explain most of the changes in MHW properties, at least over the satellite record. The analysis of long-term in situ data from six monitoring stations showed a consistent increase in MHW frequency and duration over the 20th century. The study extended the global record to the early 20th century using monthly gridded SST data sets, revealing a global average increase in MHW frequency and duration. The results indicate that the warming trend has accelerated over the past century, with significant increases in MHW frequency and duration. The study highlights the need for improved monitoring and understanding of MHWs to better predict their impacts on marine ecosystems and human societies.Marine heatwaves (MHWs) have increased significantly over the past century, with global average frequency and duration rising by 34% and 17%, respectively, leading to a 54% increase in annual MHW days. This trend is largely explained by rising mean ocean temperatures. The study used satellite and in situ data to analyze MHW characteristics from 1925 to 2016, revealing significant centennial-scale increases in MHW properties and variability on interannual to multidecadal time scales. The satellite period (1982–2016) accounts for a larger portion of this increase than in situ records. The El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) each contribute to regional and global variations in MHWs. The study also found that changes in mean SST alone can explain most of the changes in MHW properties, at least over the satellite record. The analysis of long-term in situ data from six monitoring stations showed a consistent increase in MHW frequency and duration over the 20th century. The study extended the global record to the early 20th century using monthly gridded SST data sets, revealing a global average increase in MHW frequency and duration. The results indicate that the warming trend has accelerated over the past century, with significant increases in MHW frequency and duration. The study highlights the need for improved monitoring and understanding of MHWs to better predict their impacts on marine ecosystems and human societies.