Body composition (BC) is a valuable indicator of health and performance. Air-displacement plethysmography (ADP) is a popular, efficient, and reliable method for assessing BC. A-mode ultrasonography (AUS) is another method, and its affordability, mobility, and practicality make it a plausible device for industry and health professionals. This study assessed the cross-sectional and change relationships and agreements between AUS and ADP from a 12-week resistance exercise intervention designed to increase fat-free mass (FFM). BC was measured by both modalities pre/post-intervention. Twenty-nine normal weight males (age: 20.6 ± 2.2 years) participated. Both modalities tracked changes in FFM and % body fat (%BF) ([delta] ADP = 1.7 ± 1.5 kg, -0.4 ± 2.5%; [delta] AUS = 2.4 ± 1.9 kg, -1.4 ± 2.1%; main effects for time p < 0.005, respectively), with no differences between modalities (interactions p > 0.05), and were moderately significantly correlated (r = 0.58 and r = 0.60, p < 0.01). Bland-Altman plots revealed no proportional biases. These devices tracked changes in FFM and %BF similarly over time. Future research should assess AUS at different levels of adiposity.