BACKGROUND: Gastroparesis is a delay in gastric emptying without mechanical obstruction, lacking a clear pathophysiological mechanism, but with multiple histological abnormalities including loss of interstitial cells of Cajal, which may alter slow waves. We can assess slow waves with electrogastrography. OBJECTIVES: To determine the prevalence and range of abnormalities in gastric slow waves in adults with gastroparesis using electrogastrography. METHODS: We systematically searched Medline, Embase, LILACS, Web of Science, and Cochrane Register of Controlled Trials. We included studies with patients older than 18 years with gastroparesis, assessed using electrogastrography. We evaluated the percentage of duration of the recording in which the dominant power was in normogastria, tachygastria, and bradygastria; dominant frequency; power ratio; change in post-stimulus dominant power; and dominant frequency instability coefficient. Methodological quality was assessed using the Joanna Briggs Institute tool. Data were synthesized using narrative summary and meta-analysis. RESULTS: A total of 3730 articles were reviewed, including 31 articles with 1545 patients and 340 controls. Compared to controls, gastroparetics patients had less normogastria (fasting: 50.3% versus 65.8%) (post-stimulus: 54.3% versus 66.5%), more bradygastria (fasting: 37.7% versus 13%) (post-stimulus: 31.9% versus 16.3%), and more tachygastria (fasting: 16.1% versus 4.6%) (post-stimulus: 18.3% versus 5.2%). Gastroparetics had less change in post-stimulus dominant power (1.45 dB versus 5.03 dB) and less power ratio (1.4 versus 5.26). CONCLUSIONS: Gastroparetic patients present abnormalities in the frequency and changes in the post-stimulus power of slow waves, possibly secondary to a reduced number of interstitial cells of Cajal, as described in these patients.