Map from Google - Rimini

In Rimini, and in the neighboring towns of the Adriatic Coast, such as Riccione and Cattolica, the climate is semi-continental, with fairly cold winters, and hot and sunny summers, a little stuffy, but tempered by the breeze. The city is located outside of the Po Valley and on the coast, but enough to the north to be affected, during cold periods, by the Bora, a cold wind that blows in gusts from the plains of Eastern Europe, especially in the cold half of the year. The city, however, is also subject to the Sirocco, or more precisely to the Garbino, a mild or hot southwest wind that descends from the mountains, but that may come originally from Africa.
Here are the average temperatures.
Average temperatures - Rimini
RiminiJanFebMarAprMayJunJulAugSepOctNovDec
RiminiJFMAMJJASOND
Min (°C)114712161918151162
Max (°C)891317222629292520139
Min (°F)343439455461666459524336
Max (°F)464855637279848477685548

Precipitation is moderate, given that it amounts to 645 millimetres (25.5 inches) per year; the rains are well distributed over the seasons, even though there is a maximum in autumn. However, summer is the season in which the rains are more rare; in this season, and in late spring, they can occur in the form of thunderstorms.
Here is the average precipitation.
Average precipitation - Rimini
RiminiJanFebMarAprMayJunJulAugSepOctNovDecYear
RiminiJFMAMJJASONDY
Prec. (mm)404550555055455070756545645
Prec. (in)1.61.822.222.21.822.832.61.825.4
Days66677646787776

Winter, from December to February, is cold enough, although it is less cold than in the Po Valley. However, the wind can increase the feeling of cold, especially when the Bora blows. Snowfalls are fairly common, in the sense that you can see some flakes in most years, however, they are more rare than in the interior of Romagna, and are generally not abundant. In any case, there can be cold spells with temperatures around 0 °C (32 °F) even during the day, a cold wind blowing from the sea, and brief snowfalls, also in the form of showers or thunderstorms. Frosts at night are quite common, but usually not remarkable, except during the periods of most intense cold, when the temperature can drop to -10 °C (14 °F) and below.
There's no shortage of rainy days, although the winter is the least rainy season.
When the south wind blows, the temperature becomes milder, and in the daytime it can reach and sometimes exceed 15 °C (59 °F).
In periods of high pressure, fog can form.

Spring in Rimini is a quite unstable season, especially at the beginning, in March and in the first half of April, when the first mild days alternate with the return of the cold, brought by the Bora. However, as the weeks pass, the weather becomes nicer, especially from mid-April to late May, when there are many sunny days, with mild or pleasantly warm temperatures during the day.
However, Atlantic disturbances, able to bring rainfall, are quite frequent.

Summer, from June to August, is hot and humid, and generally sunny. During the day, the sea breeze usually makes the heat bearable, while nights are sultry because of the humidity coming from the sea. The hottest periods occur when the south wind blows, because in these cases the tempering effect of the sea disappears, so that the temperature can reach 35/37 °C (95/99 °F).
Sometimes, Atlantic disturbances able to bring cool and rainy weather can pass in summer as well, perhaps accompanied by gusts of Bora, especially in June and late August, or thunderstorms can break out in the afternoon and evening, but for long periods the weather is hot and sunny.

Autumn offers several nice days in September and sometimes in early October, and then quickly becomes gray and rainy enough. The first cold days usually occur in November. Overall, autumn is the wettest season of the year.

The sunshine amount in Rimini is not high from mid-October to February, when sunny days are quite rare. However, the sun quite often shines in spring, and it is very common in summer, except for the few rainy days or when sunny days are interrupted by thunderstorms in the afternoon.

Rimini, beach and umbrellas

The sea in Rimini is warm enough for swimming from July to September, while in June it is still a bit cool.
Here are the average sea temperatures.
Sea temperature - Rimini
RiminiJanFebMarAprMayJunJulAugSepOctNovDec
RiminiJFMAMJJASOND
Sea (°C)121111131822252623201614
Sea (°F)545252556472777973686157

When to go

The best time to go to Rimini and the Adriatic coast for beach holidays, is summer, from June to August, with a preference for July and August, which is the hottest and sunniest, and when the sea is warmest as well.