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Spawning season and habitat of invasive smallmouth bass were studied for two seasons in the Chikuma River, Nagano, Japan. Smallmouth bass spawning started in early May, when required water temperatures were met, and continued as long as spawning was not disrupted by high water levels. Spawning sites were found in patches throughout the study area that had significantly different physical characteristics than typical stream habitat features. Utilized spawning sites had low current velocity, were relatively shallow, and had pebble-sized substrate. Individual nest variables showed correlation with nesting male size as larger males constructed larger and deeper nests in faster flowing and deeper areas of the river. Nest guarding rates were low while egg presence rates were high compared to the native range, which may indicate that factors such as favorable water levels and water temperature and prevalence of spawning habitat facilitate spawning and the spread of smallmouth bass in the Chikuma River.