Introduction: Because of the the complex physiopathology of spasticity, it is distinguished as one of the most significant positive clinical signs of upper motor neuron syndrome, constituting a clinical feature that has great impact in the neurorehabilitation setting. Thus, the current study aimed to determine the prevalence, onset, evolution, and prediction of spasticity after a stroke. Materials and Methods: A correlational, longitudinal design was used. A total of 136 patients were evaluated at the following times: 10 days (T1), 3 months (T2), and 12 months (T3) poststroke. The initial evaluation included sociodemographic and clinical data (T1). Muscle tone was measured (T1, T2, and T3) using the Modified Ashworth Scale. Results: The prevalence of poststroke spasticity in the elbow was 37.5% at T1 and 57.4% at T2 and T3. Among patients with motor damage, the onset of spasticity occurred at T1 in 44.7%, between T1 and T2 in 23.7%, and between T2 and T3 in 0.9%. Significant predictors of the alteration in muscular tone for at least two of the evaluation times were ethnic self-classification, type, area, extent of stroke, and number of sessions. Conclusions: Spasticity onset occurs during the first 10 days after a stroke. More clinical than sociodemographic variables predicted spasticity.