The objective of this study was to increase the α-linolenic acid (ALA) concentration in milk fat by feeding a rumen protected flax-based (RPF) supplement. Eight lactating Holstein cows at 194 ± 16.3 DIM were randomly assigned to 4 treatment sequences in replicated 4 × 4 Latin squares with 16-d periods. The 4 treatments were 0, 0.9, 1.8, and 2.7 kg of RPF (26.6% protein and 31.9% lipid, of which 49.9% was ALA) added to a base ration (3% lipid DM basis) daily. Milk samples were collected on d 15 and 16 of each period. Milk components were assessed by near-infrared analysis and fatty acids by gas chromatography. Data were analyzed using a generalized linear mixed model with cow nested within period as the random effect and treatment, period, and treatment by period interactions as the fixed effects. A β distribution was assumed for the proportion fatty acid data and a normal distribution was assumed for dry matter intake, milk yield, and milk components. RPF did not affect dry matter intake, milk yield, or most milk components. Milk fat concentration tended to increase (P = 0.02) from 3.75% to 3.91% and milk urea nitrogen concentration tended to decrease (P = 0.08) from 14.2 to 11.9 mg/dL as the amount of RPF supplement added to the diet increased. Many fatty acids in milk were altered by the RPF dose showing a generally decreasing trend in short- and medium-chain saturated fatty acids and increasing trends in long-chain fatty acids. Compared with the diet with no added RPF (ALA concentration 0.53 g/100 g total fatty acids), RPF added at 0.9, 1.8, and 2.7 kg/d increased (P < 0.0001) the concentration of ALA in milk to 1.43, 2.14, and 2.77 g/100 g total fatty acids, respectively. The n-6-to-n-3 fatty acid ratio was lowered (6.6 to 1.3) as the amount of RPF supplement added to the diet increased. This novel rumen protected ALA source enriched milk ALA greater than any previous method.