Reading motivation significantly influences the academic success of American college students. Existing literature often treats American students as a homogeneous group and overlooks the impact of diverse ethnic backgrounds on reading motivation. To address this gap, the present study investigated the relationships among reading motivation, reading amount, and reading comprehension in a sample of 1360 American college students representing three ethnic groups: White American, African American, and Hispanic American. Additionally, we explored the role of reading amount as a mediator in the relationship between reading motivation and reading comprehension, and assessed the magnitude of these effects across the three ethnic groups. Path analysis for each ethnic group revealed a direct effect of expectancy on reading comprehension for all groups. Furthermore, indirect effects of value and cost on reading comprehension were observed among White American and Hispanic American college students, whereas African American college students exhibited a direct effect of cost on reading comprehension. Multigroup path analyses showed similar magnitudes of direct and indirect effects across the three ethnic groups. Our findings provide new evidence of both commonalities and differences in reading motivation among American college students from different ethnic backgrounds.