Purpose The first aim of this study was to evaluate changes in the stiffness of the medial gastrocnemius muscle (GM) after a botulinum toxin A (BoNT-A) injection in children with cerebral palsy (CP) using shear wave elastography (SWE). We also wanted to investigate the usability of SWE for evaluating spasticity in a clinical setting. The second aim of this study was to show how treatment of the gastrocnemius muscle spasticity caused a change in the elasticity of the anterior tibial (TA) muscle. Methods Twenty-four pediatric patients diagnosed with a spastic type of CP, who were scheduled to receive a BoNT-A injection in the gastrocnemius muscle, were included in the study. There was a total of 43 lower extremities to evaluate, and muscle stiffness was measured before the injection and a month post injection using SWE. The physiatrist evaluated muscle spasticity using the Modified Ashworth Scale (MAS) and the Modified Tardieu Scale at about the same time. Results SWE values of the GM (pre-BoNT-A: 45.9 +/- 6.5 kPa, post-BoNT-A: 25.0 +/- 5.7 kPa) decreased significantly post BoNT-A injection (P < 0.01). SWE measurements of the GM had positive correlations with MAS, V1X, V3X, and R2-R1 (P < 0.01); and negative correlations with R2 and R1 (P < 0.05). SWE values of the TA muscle (pre: 36.9 +/- 7.9 kPa, post: 28.4 +/- 5.2 kPa) decreased significantly (P < 0.01). Conclusion Quantitative measurement of muscle stiffness using SWE may provide important information for the evaluation of spasticity and treatment efficiency in pediatric CP patients.