Extraction of bitumen from mined oil sands ores produces enormous volumes of tailings that are stored in settling basins (current inventory ≥840 million m3). Our previous studies revealed that certain hydrocarbons (short-chain n-alkanes [C6–C10] and monoaromatics [toluene, o-xylene, m-xylene]) in residual naphtha entrained in the tailings are biodegraded to CH4 by a consortium of microorganisms. Here we show that higher molecular weight n-alkanes (C14, C16, and C18) are also degraded under methanogenic conditions in oil sands tailings, albeit after a lengthy lag (∼180 d) before the onset of methanogenesis. Gas chromatographic analyses showed that the longer-chain n-alkanes each added at ∼400 mg L–1 were completely degraded by the resident microorganisms within ∼440 d at ∼20 °C. 16S rRNA gene sequence analysis of clone libraries implied that the predominant pathway of longer-chain n-alkane metabolism in tailings is through syntrophic oxidation of n-alkanes coupled with CO2 reduction to CH4. These studies demonstrating methanogenic biodegradation of longer-chain n-alkanes by microbes native to oil sands tailings may be important for effective management of tailings and greenhouse gas emissions from tailings ponds.