Walnut flavor is considered an important determinant of eating quality and processing suitability. However, the volatile aroma profiles and key aroma-active compounds of walnuts from different cultivars and production regions have not been fully elucidated. In the present study, 12 walnut cultivars collected from four major production regions in China, including Dali in Yunnan, Hotan in Xinjiang, Zanhuang in Hebei, and Shangluo in Shaanxi, were used to characterize their volatile compositions and identify the key aroma-active compounds. Volatile compounds were detected by headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry (HS-SPME-GC–MS). Differences in aroma profiles were evaluated using single-factor experiments, principal component analysis (PCA), orthogonal partial least-squares discriminant analysis (OPLS-DA), and relative odor activity value (ROAV) analysis. Six classes of volatile compounds, including alcohols, aldehydes, esters, aromatic hydrocarbons, olefins, and heterocyclic compounds, were identified. Aldehydes were found to be the predominant compounds, accounting for 37.2% of the total relative abundance. The 12 walnut cultivars were classified into three distinct groups by PCA. Nine discriminant compounds were screened, namely 1-pentanol, (R)-2,3-butanediol, D-limonene, ethylbenzene, o-xylene, hexanal, 3-methyl-1-butanol, sec-butyl acetate, and 2-methyl-1-butanol. Among these compounds, hexanal was characterized by a relatively high ROAV and was identified as a major contributor to raw-oil-like, apple-like, and grassy notes. Significant differences in aroma profiles were observed among walnut cultivars and production regions, and hexanal was identified as a key aroma-active compound. These findings may provide a theoretical basis for walnut cultivar identification, geographical origin traceability, quality evaluation, and processing optimization.