Flax seed (Linum usitatissimum L.) has been reported to provide several health benefits which include, reduction of cholesterol, protection against several cancers and improvement in blood sugar. For this benefit, the consumption of 62 g of flax seed per day is recommended. Acid tolerance is an important probiotic characteristic. The objective was to determine the effect of various amounts of flax seed powder on the acid tolerance of Streptococcus thermophilus. M17 broth of several pH values (1, 1.5, and 2) were prepared without and with flax seed powder incorporated at 62 g/L. Flax seed powder was dispersed using a magnetic stirrer. After sterilizing and cooling the M17 broths, freshly thawed Streptococcus thermophilus ST-M5 was incorporated and incubated at 42°C for 4 h. Samples were drawn at 2, 2.5, 3, 3.5, and 4 h, serially diluted and pour plated on M17 agar. Pour plates were aerobically incubated at 42°C for 48 h and colonies were counted. Samples were plated in duplicate. Entire experiment was replicated 3 times. Data were analyzed using PROC ANOVA of SAS, with means being separated using Fisher Least Significant Difference test. Mean counts of control pH 2, 1.5 and 1.00 and flax seed pH 2, 1.5 and 1.00 were 9.23 ± 0.25; 8.50 ± 0.43; 4.01 ± 0.54; 9.28 ± 0.26; 8.82 ± 0.37 and 4.39 ± 0.41 log cfu/mL. Flax seed counts at pH 1.5 and 1 were significantly (P < 0.05) higher than control. Flax seed enhanced the acid tolerance of Streptococcus thermophilus ST-M5.