September, for most people. On the Split Marjan record for 1948 to 2024, September averages 17 days above 25°C and October averages one. The sea peaks in August at 23.6°C and is still 21.7°C in September. Croatia’s season stops rather than fades.
The one number that decides it
Most best-time guides describe a gentle taper from summer into autumn. The Croatian record does not show one. The Split Marjan station has been measuring since 1948; across that record September carries 17 days with a maximum above 25°C and October carries one. That is a 94 per cent fall in four weeks, and it is the single most useful fact anyone planning a Croatian trip can hold.
Days above 25°C per month at Split Marjan. Source: DHMZ (meteo.hr), Split Marjan station, 1948–2024.
The practical reading is that the shoulder season on the way out is one month long, not three. April and May are a genuine ramp; September is a cliff edge with a view. Plan the trip so that the last swimming day falls in September rather than hoping October will hold.
Why August and September are the real comparison
The sea lags the air by about six weeks and peaks in August, which is why the standard advice to “go in June instead” is wrong for anyone who wants to swim. September has warmer water than June at every station DHMZ measures one for, and only two degrees less than August.
| Month | Split sea °C | Hvar sea °C | Dubrovnik sea °C | Split days ≥25°C | What is sailing |
|---|---|---|---|---|---|
| June | 21.1 | 20.8 | 21.1 | 22.0 | Shoulder timetable, season tariff. Line 634 at 20 a day. |
| July | 23.2 | 22.9 | 22.9 | 29.5 | Peak from 3 July. Line 634 at 22 a day; line 9807 full route. |
| August | 23.6 | 23.5 | 23.3 | 29.0 | Peak to 30 August, then shoulder. Season tariff throughout. |
| September | 21.7 | 22.4 | 21.8 | 15.4 | Shoulder. Season tariff to 27 September, then off-season. |
| October | 19.3 | 20.6 | 19.5 | 1.3 | Off-season tariff. Line 634 back to 14 a day. |
The trade between them is not the one most guides describe. June has the better land figures — 22.0 days above 25°C at Split against September’s 15.4 — and September has the better water at every station that measures one. What June also has is a rising curve rather than a falling one, which matters only if the trip runs over a fortnight.
September · Split (Marjan)
Sea 21.7°C at Split and 22.4°C at Hvar, warmer than June, with 15.4 warm days. Fares hold at season rates to 27 September: quieter, not cheaper.
Two things move independently on that slider. The climate row falls off after September; the ferry row changes on four dates that have nothing to do with the weather. A month that looks fine on the metrics can still be the wrong month if the boat you need has stopped.
Booking a shoulder trip to save money on the ferries
The tariff season runs 29 May to 27 September without a break. A trip in the third week of September pays exactly the same fare as one in August — €52.00 for a car under 5 m to Vis, €10.00 on foot to Hvar — while running on a reduced timetable.
The saving is real only either side of those two dates. Late May before the 29th, and the very end of September after the 28th, are the two windows where off-season fares coincide with a usable sea. Everything in between is priced the same.
So which month, for whom
The honest answer depends on two things: what you are actually here to do, and whether you are tied to school holidays. Those two questions settle it faster than any month-by-month guide.
What is the trip for
How much choice do you have
What September actually looks like →
Whichever way that lands, the region matters nearly as much as the month. Dubrovnik keeps 16.7 warm days in September and Rijeka keeps 10.4; in October Rijeka takes 203.9 mm of rain and Split 82.1 mm. Weight a shoulder-season trip south. Full station-by-station figures are on Croatia weather by month.
What to decide before you book
- You want the best all-round month. September: 17 warm days on the long record and a 21.7°C sea.
- You want the warmest sea. August, at 23.6°C. Nothing else in the year comes close.
- You want it quiet and cheap. Late September into October, once the off-season tariff returns on 28 September.
- You are considering October for beach weather. Do not. One day above 25°C at Split Marjan in the whole month.
Find flights and hotels for your dates
What do you need to work out next?
The month is settled. These three decide the rest.
Where to stay
Which coast suits a shoulder trip?
Dubrovnik keeps 16.7 warm days in September; Rijeka keeps 10.4.
Getting around
Do the islands still connect in September?
Line 634 runs 20 sailings a day, but line 9807 lost three ports on 30 August.
When to go
What do the other months look like?
Eight DHMZ stations, and a sea temperature published for three of them.
Planning
Now I know when, so where?
Ferry season tariffs run 29 May to 27 September on every line checked.
Climate data: DHMZ (meteo.hr), climate normals 1971–2000. Climate data: DHMZ (meteo.hr), Split Marjan station, 1948–2024. Ferry data: Agencija za obalni linijski pomorski promet (ZOLPP), 2026 state coastal line register and per-line timetables.