Urbanization　has　become　one　of　the　most　important　human　activities　modifying　the　Earth＇s　land　surfaces;　and　its　impacts　on　tropical　and　subtropical　cities　(e.g.,　in　South/Southeast　Asia)　are　not　fully　understood.　Colombo;　the　capital　of　Sri　Lanka;　has　been　urbanized　for　about　2000　years;　due　to　its　strategic　position　on　the　east-west　sea　trade　routes.　This　study　aims　to　investigate　the　characteristics　of　urban　expansion　and　its　impacts　on　land　surface　temperature　in　Colombo　from　1988　to　2016;　using　a　time-series　of　Landsat　images.　Urban　land　cover　changes　(ULCC)　were　derived　from　time-series　satellite　images　with　the　assistance　of　machine　learning　methods.　Urban　density　was　selected　as　a　measure　of　urbanization;　derived　from　both　the　multi-buffer　ring　method　and　a　gravity　model;　which　were　comparatively　adopted　to　evaluate　the　impacts　of　ULCC　on　the　changes　in　land　surface　temperature　(LST)　over　the　study　period.　The　experimental　results　indicate　that:　(1)　the　urban　land　cover　classification　during　the　study　period　was　conducted　with　satisfactory　accuracy;　with　more　than　80%　for　the　overall　accuracy　and　over　0.73　for　the　Kappa　coefficient;　(2)　the　Colombo　Metropolitan　Area　exhibits　a　diffusion　pattern　of　urban　growth;　especially　along　the　west　coastal　line;　from　both　the　multi-buffer　ring　approach　and　the　gravity　model;　(3)　urban　density　was　identified　as　having　a　positive　relationship　with　LST　through　time;　(4)　there　was　a　noticeable　increase　in　the　mean　LST;　of　5.24　°C　for　water　surfaces;　5.92　°C　for　vegetation;　8.62　°C　for　bare　land;　and　8.94　°C　for　urban　areas.　The　results　provide　a　scientific　reference　for　policy　makers　and　urban　planners　working　towards　a　healthy　and　sustainable　Colombo　Metropolitan　Area.　©　2019　by　the　authors.