The three-dimensional non-hydrostatic mesoscale model, OMEGA (Operational Multiscale Environment model with Grid Adaptivity), is utilized to investigate the thermally driven local flows and their interaction with each other over the province of Istanbul, Turkey. Idealized case simulations are conducted in order to describe the contribution of sea–land breezes and urban heat island (UHI) circulation to the local flow over the city. The city of Istanbul is located between two water bodies, Black Sea in the north and Sea of Marmara in the south. A convergence zone is observed at about 1000 LST over the region due to the merge of two sea breezes that develop at both sides and advance towards inland areas. Investigation further indicates that the other geographic features of the city have substantial effects on the local flow. The Bosphorus Strait, which connects Black Sea to Sea of Marmara and divides the city into two parts, channels the flow. Although not significantly high and steep, the topography enhances the flow strength. Besides, urbanized regions in the south, by generating UHI circulation, prevent the inland penetration of the sea breeze that develops in the south. The numerical simulations reveal that the large-scale wind direction controls the inland penetration of the sea breeze. One of the outstanding features of the local flows is that all hypothetical simulations including the Bosphorus indicate a strong fan-like airflow in the immediate south of the strait. The flow is more intense in the northerly prevailing wind case. It is also found that stronger large-scale flows (e.g. 2.5 and 10 m s−1) do not allow the formation of a sea breeze over the region. A warmer sea surface than land surface (about 2 °C) makes changes in the duration and extension of the land–sea breeze circulation. The duration of a land breeze circulation is long, and the southerly sea breeze is onset close to the south coast under the northeasterly flow conditions.