September in Istanbul features warm to comfortable afternoons with comfortable to cool evenings. About 4-5 days, mostly early in the month, will be humid.
Daytime high temperatures tend to be around 26-27 C (79-81 F) early in the month, cooling down to around 22-23 C (72-73 F) by the end of the month.
A few of the warmer afternoons, especially early in the month, can reach up near 31 C (88 F). The record high temperature in Istanbul during September is 39.6 C (103.3 F).
Late night and early morning lows tend to be around 18-19 C (64-66 F) early in the month, falling to around 15 C (59 F) near the end of the month.
A few of the cooler mornings, especially near the end of the month, can dip down to around 12 C (54 F). The record low temperature in Istanbul during September is 3.1 C (37.6 F).
Sky and Rain
You can expect 23-24 sunny, or at least partly sunny days this month with 6-7 mostly cloudy days. It is more likely to have a sunny day early in the month than near the end.
About 5-8 days will record some rain with rainfall slightly more likely towards the end of the month.
Wind speeds during this month will be mostly gentle to moderate at 12-28 kph (8-18 mph) but can briefly be fresh at 29-38 kph (19-24 mph).
The beaches near Istanbul lie mostly along the shores of the Sea of Marmara and along the Bosporus Strait.
The sea temperature starts to cool during September from about 23 C (74 F) early in the month down to about 21 C (70 F) near the end of the month.
What to Wear in Istanbul during September?
During September you should pack clothes for both summer and early fall. This can include light airy cotton clothing for the warmer days.
A light sweater and windbreaker will come in handy for the cooler evenings and early morning hours.
Sundresses and knee-length skirts/shorts and t-shirts and jeans are ok but also include some long-sleeve shirts and pants for the cooler days.
When visiting a Mosque, women are required to cover their hair, upper arms, shoulders, and knees.
Things to Do in Istanbul in September
The weather should still be very nice on most days this month, so you might want to think about taking a pay-what-you-wish Istanbul walking tour.
Ocean Weather Services