In the present study, a novel water index has been proposed for mapping of open surface water in multispectral images and named as open surface water index (OSWI). The proposed index has been applied on Landsat-8 data to delineate the water area and the results are compared with those of existing water indices. The index is also applied on Sentinel-2A data and corresponding water pixels are extracted. The results have been validated with the help of classification as well as density slicing based methods on multisensory data. The preliminary results show that the proposed index is capable of identifying smaller water regions which remain unidentified by other indices.