Background/Objectives: Overweight and obesity (OW/OB) in immigrant children is a complex multifactorial issue. This work aims to present the OW/OB profile and lifestyle habits of immigrant children and compare them with their native counterparts. Methods: A cross-sectional study (ASOMAD) was conducted in a representative sample of children aged 8-12 years in Madrid from 2020 to 2023. Parental origin, physical activity (PA), screen time, adherence to Mediterranean Diet, sleep, and body composition were assessed by validated methods. OW/OB was determined according to IOTF guidelines. T-tests, chi-square tests, and two-way ANOVA tests were applied based on the variables' characteristics. Results: A total of 587 children (54% boys, aged 9.54 +/- 1.19 y), 33.2% immigrants, were studied. OW/OB rate was higher in immigrants (39.7% boys and 37.4% girls) than in natives (18.0% boys and 17.7% girls) (p < 0.05). Immigrant boys were 30.64 +/- 8.39 min/day less active than natives (p < 0.001). Both male and female immigrant children devoted considerably more weekday hours to screen time than natives (2.76 +/- 2.75; 2.02 +/- 2.47 vs. 2.09 +/- 2.29; 1.32 +/- 1.38; p < 0.05, respectively). Immigrant children consumed significantly less fish and olive oil and more pasta or rice almost every day (5 or more/week), ate at fast-food restaurants, consumed fewer dairy products and baked goods or pastries for breakfast (p < 0.05), and exhibited worse diet quality compared to Spanish ones. Conclusions: Immigrant children exhibited a higher prevalence of OW/OB, higher screen time, lower PA time, and ate less fish and olive oil and more pasta than natives. Additional research is required to explore the causes of these issues and enhance the lifestyle within this group.