From NOAA’s Global Climate Report – May 2021:
The May 2021 global surface temperature was 0.81°C (1.46°F) above the 20th century average of 14.8°C (58.6°F). This value tied with 2018 as the sixth warmest May in the 142-year record. May 2021 was also the 45th consecutive May and the 437th consecutive month with temperatures, at least nominally, above the 20th century average.
According to the May 2021 temperature percentile map, the month of May was characterized by much-warmer-than-average temperatures across parts of northern, western, and southeastern Asia, Africa, northern South America and across parts of the Pacific, Atlantic, and the Indian oceans. The most notable warm temperature departures from average were observed across parts of western and northern Asia and northern Africa, where temperatures were at least 2.5°C (4.5°F) above average. Record-warm May temperatures were observed across parts of northern Africa, western Asia, and small areas across the Atlantic Ocean and the South Pacific Ocean. This encompassed only 3.0% of the world’s surface with a record-warm May temperature—the tenth highest May percentage for record-warm May temperatures since records began in 1951.
The time series data can be found near the top of the page.