Fruit spine density is an important commercial trait for cucumber (Cucumis sativus L.). Most North China-type cucumbers that are grown over large areas have a dense-spine phenotype, which directly affects the appearance quality, storage, and transportation of the fruits. Here, we isolated a novel few spines mutant (fs2) from the wild-type (WT) inbred line WD1, a North China-type cucumber with high density fruit spines, by an ethyl methanesulfonate (EMS) mutagenesis treatment. Genetic analysis revealed that the phenotype of fs2 is controlled by a single recessive nuclear gene. We fine-mapped the fs2 locus using F₂ and BC₁ populations (1,802 and 420 individuals, respectively), which showed that the candidate gene of FS2 (Csa4G652850) encodes an ARID-HMG transcription factor containing an AT-rich interaction domain (ARID) and a high mobility group box domain (HMG). One SNP (C to T) and one InDel (a 40-bp deletion) in the coding region of FS2 result in amino acid variation and premature translation termination in the fs2 mutant, respectively. FS2 was found to be highly expressed in the apical buds and young ovaries. In addition, experiments suggest that FS2 participates in the regulation of fruit spine initiation by activating the expression of the Tril gene in cucumber. This work provides not only an important reference for understanding the molecular mechanisms of fruit spine development but also an important resource for fruit appearance quality breeding in cucumber.