Microalgae cultivation in dairy wastewaters is of major interest for nutrient removal and sustainable microalgal biomass production. This biomass is a profitable raw material to produce biofertilizers and biocompounds for use in agriculture and food and pharmaceutical industries. This study aimed to compare biomass production, morphology, and phytohormone levels in Spirulina platensis (UTEX LB1926) grown on dairy wastewaters, in comparison to Zarrouk medium. An extraction method from lyophilized biomass was developed to detect and quantify the endogenous phytohormones Indole-3-acetic acid (IAA), Phenylacetic acid (PAA), Salicylic acid (SA), Jasmonic acid (JA), Abscisic acid (ABA), Gibberellin A1 (GA1), Gibberellin A4 (GA4), Indole-3-butyric acid (IBA), 1-Aminocyclopropane-1-carboxylic acid (ACC), 6-Benzylaminopurine (BAP), and Kinetin (KA) in S. platensis, cultivated in dairy wastewater and cheese whey, from a local dairy industry. Phytohormones were generally higher in S. platensis cultivated in dairy wastewaters than in a synthetic medium. The concentrations of phytohormones widely varied with the culture medium and the lighting conditions. Low light intensity significantly promoted the filaments of S. platensis to grow longer and thicker in wastewater, especially in cheese whey, enhancing biomass harvest. The cultivation of S. platensis in dairy wastewaters allows the production of both biomass and phytohormones, at low cost, while treating these waters.