A new barnyard millet variety, ‘Matnajik,’ was developed by the Chungcheongbukdo Agricultural Research and Extension Services from 2014 to 2023 through gamma-ray mutation breeding. Morphologically, ‘Matnajik’ exhibits a semi-straight plant type, conical green panicles, grayish-white seeds, yellowish-brown dehusked grains, and a non-glutinous endosperm. Its heading date is July 29, classifying it as a medium-maturing variety that heads approximately 11 days later than the control variety, ‘Borajik.’ Notably, ‘Matnajik’ has a large panicle type; its average culm length is 138 cm, and its panicle length is 23 cm, both of which are significantly longer than those of the control. In the yield trials conducted from 2022 to 2023, the average grain yield of ‘Matnajik’ was 252 kg⋅10a^-1, a 13% increase over ‘Borajik’ (224 kg⋅10a^-1). Furthermore, physiochemical and biological activity analyses revealed that ‘Matnajik’ contains higher levels of crude ash and essential minerals, including potassium, calcium, and magnesium. Specifically, it showed significantly higher total polyphenol and flavonoid contents as well as greater radical-scavenging activity than the control, indicating its high value as a functional food material. In conclusion, ‘Matnajik’ is a promising variety with a large panicle, high yield, and enhanced functional properties, and it is expected to be widely utilized as a new specialized cash crop in the Chungbuk region (Registration No. 10814).