Interseeding alfalfa into rangelands has been assessed for decades as a method of range improvement to increase forage production and forage quality for livestock. Research was initiated in 2001 to examine the long term effects of interseeding yellow-flowered alfalfa (Medicago sativa ssp. falcata) on northern mixed-grass rangelands. Forage production and forage quality parameters were assessed on sites interseeded in 1965, 1987, and 1998 and compared to adjacent native rangelands. Live aboveground biomass for the 1965, 1987, and 1998 interseeded sites was 68, 143, and 42% higher, respectively, compared to their native control areas. Alfalfa aboveground biomass accounted for 1 489 of the 2 969 kg·ha⁻¹ live biomass harvested from the 1965 interseeded site, 1 940 of the 2 744 kg·ha⁻¹ on the 1987 interseeded site, and 796 of the 2 322 kg·ha⁻¹ on the 1998 interseeded site. Increased soil N resulting from N fixation by the alfalfa significantly increased the crude protein (CP) content of several native species, whereas the alfalfa itself provided forage with 16 to 18% CP. Alfalfa had higher protein degradability and provided higher concentrations of calcium (Ca), potassium (K), and magnesium (Mg) than the native rangeland grasses. This research has shown that the practice of interseeding yellow-flowering alfalfa into rangelands is sustainable over decades and will increase forage production and improve nutritive value of forage in the northern Great Plains.