Portorož has a Mediterranean climate that makes it genuinely appealing across much of the year, though the character of the visit changes dramatically by season.
Summer (June–August) is peak season: the beach is crowded, prices are at their highest, and accommodation books up weeks in advance, especially in July and August when Central European tourists arrive en masse. The sea temperature reaches 24–26°C and the days are long and reliably sunny. The Portorož Auditorium runs an active summer programme of concerts and events.
Autumn (September–October) is arguably the best time to visit. The crowds thin out sharply after mid-September, but the sea stays warm enough to swim well into October. The light is extraordinary at this time of year — the kind of clear Mediterranean autumn light that photographers travel for. The Slovenian Film Festival (Festivala slovenskega filma) takes place in September, bringing an interesting cultural dimension to the town.
Spring (April–May) is pleasant for walking, cycling, and visiting the salt pans — the Sečovlje Salina Nature Park is particularly rewarding in spring when migratory birds are passing through. The sea is still too cold for comfortable swimming, but the promenade is walkable and the hinterland is green.
Winter (November–March) sees many restaurants and cafes close or reduce hours. The spa hotels remain open year-round, and visiting for thermal treatments in the quiet off-season has a particular appeal — lower prices, no crowds, and the melancholy beauty of an out-of-season resort. Snowfall is rare but possible.