The aim of the study was to evaluate the effect of a diet containing extruded flaxseed (Linum usitatissimum L.) on the fatty acid composition of the loin, blood lipid parameters, and the redox status of tissues of finishing pigs. A total of 160 weaners (about 30–110 kg BW) were assigned to four experimental groups of 40 animals each (5 replicates with 8 individuals each). Group C (control) received a diet in which the fat source was soybean oil, while in groups 2FE, 4FE and 6FE soybean meal was replaced with extruded flaxseed in the amount of 2%, 4% or 6%, respectively. The diet containing extruded flaxseed reduced cholesterol levels in the blood plasma of pigs (grower 2FE and 6FE vs. C; finisher 2FE, 4FE and 6FE vs. C) and loin muscle (2FE, 4FE and 6FE vs. C). A decrease in the atherogenic LDL-C fraction and in the content of triacylglycerols was also noted in the blood plasma of grower and finisher pigs receiving flaxseed (2FE, 4FE and 6FE vs. C). The beneficial effects noted in the experimental pigs also included an increase in the overall content of n-3 PUFAs, especially ALA (18:3 n-3), and a reduction in the n-6/n-3 ratio. This was especially evident at 4% and 6% inclusion of flaxseed. The highest proportion of flax (6%) in the blend increased lipid peroxidation, as evidenced by the increase in the content of LOOH and MDA in the blood plasma of grower and finisher pigs. For this reason, a 4% share of flaxseed in the diet of fattening pigs seems to be optimal, while higher levels require an additional supply of exogenous antioxidants.