Nine diverse Upland cotton cultivars and germplasm lines differing in seed size were planted at two locations at Stoneville, MS in 2015, 2016, and 2017. ‘AR 9317-26’ and ‘DP 555 BG/RR’ were classified as small with a seed index (SI) < 10 g. ‘FM 832’, ‘FM 966’, and ‘MD 15’ had SI ranging from 10 to 12 g and were classified as intermediate seed size. ‘TAM 182-34 ELS’ and three other breeding lines: ‘201-2’, ‘107-1’, and ‘152-1’ had large seeds with SI > 12 g. The seeds were planted in three replications at two sites at Stoneville, MS. Data were collected on ginning energy requirement (Wh kg-1 lint), ginning rate (g lint s-1), and other agronomic and quality traits. The objectives of the test were to determine the effect of seed size on the above parameters. Statistical analyses were performed using Proc GLM. Simple Pearson’s correlation tests and regression analyses were conducted to test the relationships between SI and these traits. Covariance estimates were calculated using Proc GLIMMIX to determine the direction of linear relationships. Differences in SI were highly significant among cultivars. SI was positively and significantly correlated with ginning rate but significantly and negatively correlated with ginning energy requirement. Significant and negative relationships were observed between SI and fiber uniformity, lint yield, lint turnout, and number of seeds per kg. Significant and positive relationships were observed between SI and fiber strength, fuzz percentage, and seed surface area. Relationships among SI and micronaire, fineness, and fiber length were minor.