Field experiments were carried out on medium black soils at Agricultural Research Station, Dharwad during 2004-05 and 2005-06 under rainfed conditions to study the effect of soil and foliar application of potassium on seed cotton yield (SCY) and fibre qualities of cotton. Potassium applied as basal dose @ 40 kg/ha along with recommended nitrogen (80 kg/ha), phosphorus (40 kg/ha) and foliar potassium sprays each at early and peak boll formation stage with 1.0 % Muriate of Potash (MOP) resulted in higher SCY. Treatment with RDF + Foliar sprays with 1.0 % MOP each at early (90 DAS) and peak boll formation stage (110 DAS) recorded higher SCY of 1806 kg/ha which was significantly higher over without potassium application (1482 kg/ha). Higher net returns (Rs.19,960/ha) and B:C ratio (2.24) were obtained with RDF + twice K foliar sprays each at early and peak boll formation stage as compared to the application of N P (Rs.13,960/ha) and RDF only (Rs.17,780/ha). Higher K accumulation in different plant parts of cotton and higher K uptake was found in K applied treatments. Influence of potassium on fiber length, uniformity ratio, micronaire value was found non significant but fibre tenacity was influenced significantly. Higher fiber tenacity of 23.4 g/tex was recorded with RDF + K foliar sprays each at early (90 DAS) and peak boll formation stage (110 DAS) as compared to the treatment without potassium (22.2 g/tex) during 2005-06.