Acetolactate synthase (ALS; EC 2.2.1.6) catalyzes the first committed step in branched-chain amino acid (BCAA) biosynthesis and is the molecular target of multiple herbicide classes, including the imidazolinones. Here, we performed a genome-wide characterization of the HaALS gene family in sunflower (Helianthus annuus L.) and investigated genotype-dependent transcriptional responses to imazethapyr. A total of 11 HaALS genes were identified and classified into three phylogenetic clades (Groups A–C). All HaALS proteins contained the conserved TPP_enzyme domains (TPP_enzyme_N, TPP_enzyme_M, and TPP_enzyme_C), and purifying selection (Ka/Ks < 1) indicated strong evolutionary constraint on their core enzymatic function. Promoter analyses revealed abundant cis-regulatory elements associated with diverse stress and signaling inputs, supporting regulatory potential for herbicide-triggered transcriptional modulation. qRT-PCR analysis following imazethapyr application (0, 24, and 48 h) showed pronounced genotype-dependent expression reprogramming between the susceptible (S) and resistant (R) cultivars. In the R genotype, multiple HaALS members were strongly induced after treatment; specifically, HaALS4 reached a ~6-fold increase at 24 h and a >10-fold increase at 48 h, and HaALS11 increased to ~6–7-fold at 24 h while remaining above the baseline at 48 h; several additional paralogs exhibited intermediate induction (~3–8-fold by 48 h). In contrast, the S genotype showed limited changes (typically ~0.8–2-fold). Collectively, these findings define the evolutionary features of the sunflower HaALS family and identify herbicide-responsive paralogs that may contribute to imidazolinone tolerance, providing candidates for functional validation and molecular breeding.