Pycnanthemum tenuifolium, the narrowleaf mountainmint, slender mountainmint, common horsemint or Virginia thyme, is a plant in the mint family, Lamiaceae. It is native to eastern North America.

This is an herbaceous plant with narrow, opposite, simple leaves, on wiry, green stems. The flowers are white, borne in summer. Like most plants in the genus, the foliage has a strong mint fragrance when crushed or disturbed.