Perilla seed extract (PSE), a natural resource rich in α-linolenic acid and flavonoids, represents a promising dietary strategy to sustainably optimize rumen fermentation and improve the nutritional profile of ruminant meat. This study evaluated the effects of dietary PSE supplementation on rumen fermentation, microbiome–metabolome profiles, and subsequent meat quality in Tan lambs. Sixty 3-month-old male Tan lambs were randomly assigned to four dietary treatments (n = 15 per treatment) containing 0% (CON), 0.01% (LPSE), 0.03% (MPSE), or 0.05% (HPSE) PSE on a dry matter (DM) basis. In the rumen, the 0.03% PSE inclusion increased the propionate proportion from 20.50% to 23.80% (P-linear = 0.004) and carboxymethyl cellulase activity from 12.45 to 14.85 U/mL (P-linear = 0.007; P-quadratic = 0.045). Exploratory metagenomics showed that 0.03% PSE enriched Prevotella (18.67% to 21.06%) and Ruminococcus_E (1.20% to 2.13%), while decreasing the biohydrogenating genus Butyrivibrio compared with CON (LDA > 2, p < 0.05). These microbial shifts were accompanied by the accumulation of beneficial metabolites (e.g., small peptides and itaconic acid) and up-regulation of the pantothenate and CoA biosynthesis pathway. Consequently, the 0.03% PSE diet optimized meat quality, decreasing shear force by 12.7% (from 45.65 to 39.85 N; P-linear = 0.005, P-quadratic = 0.018) and drip loss (from 4.82% to 3.85%; P-linear = 0.022, P-quadratic = 0.015), while increasing redness (P-linear = 0.012, P-quadratic = 0.045). Furthermore, it increased meat C18:3n-3 (from 0.62% to 0.91%) and total n-3 PUFA (from 1.12% to 1.52%), while decreasing the n-6/n-3 ratio from 6.76 to 5.13 (P-linear ≤ 0.005 for all). Flavor amino acids also increased (P-linear = 0.008). These findings suggest that 0.03% PSE supplementation potentially improves lamb meat quality by favorably modulating rumen fermentation and microbe-metabolite interactions, highlighting its promise as a natural feed additive, though further validation is warranted.