Nectarine tree named ‘Sunectwentyone’

A new and distinct nectarine tree variety, prunus persica var. ‘Sunectwentyone’ that possesses a relatively low chilling requirement of 250 hours at or below 7.2° C.; heavy production of relatively large (66 mm), very firm fruit that can be harvested about May 8-15 in Kern County, Calif., (about 4 days after ‘April Glo’ nectarine (U.S. Plant Pat. No. 7305)). The nectarine fruit is bright red with bright yellow flesh that is moderately juicy, very firm, and tart-sweet with 12 brix. The plant has a relatively low tendency for split-pits, russeting, and thrips damage.