Spain is large enough that each region experiences different weather patterns and temperatures. By September the capital of Madrid is beginning to cool off after the typically brutal August heat and you can expect average highs of around 79°F/26°C.
You can expect the same highs in Barcelona, but its slowest to cool off in the southern Andalusia region, which is still hot at around 89°F/31°C. Where it gets coolest is on the northern Atlantic coast and the Basque country, which experiences highs of about 72°F/22°C and lows can dip into the high 50s (Fahrenheit).
Weather-wise, September is an ideal month because temperatures are still warm enough to make for enjoyable beach holidays. In many places, such as Ibiza, the Canary Islands, Barcelona, the Costa del Sol and Andalusia, the water will be warm enough for swimming in addition to sunbathing.
Crowds & Costs
One of the best things about traveling to Spain during any of the shoulder seasons is that prices will be lower and crowds will be thinner. This is particularly true in September. For example, you can find trans-Atlantic flights to Spain for hundreds of dollars less than they would cost during the high-season summer months. And while you likely won't save hundreds of dollars on hotels, prices should be about 20% lower than they would be during the high season.
Where to Go
The sun will be out in September, making most beaches an attractive prospect. However, if you really want to bronze under the strongest rays you can venture further south to the Andalusia region and the Costa del Sol. The beaches in Marbella will be particularly inviting as the water will be nice and warm. The same could be said for Ibiza and the Canary Islands.
If your goal isn't so much a beach holiday but rather to enjoy some great food and indoor culture, you might want to visit San Sebastian and the Basque country. The mild temps in September make strolling the waterfront with views of San Sebastian Bay an attractive prospect, but it's the city's pintxos culture that really draws the crowds. These tapas-like small plates are elevated cuisine and an integral part of Basque culture that you can find in most cafés/restaurants in the city center.
Nearby, in the city of Bilbao, you'll find the Guggenheim Museum, which is not only an architectural marvel (it was designed by Frank Gehry) but it features an impressive collection of contemporary art from not only Spain but all over the world. They feature ever-changing modern art exhibitions with different themes. Other great museums in Spain include the Prado in Madrid, and both the Picasso Museum and Museu Nacional d'Art de Catalunya, in Barcelona.
You could also visit the coastal city of Valencia, which is located about three hours south of Barcelona, and visit the City of Arts and Sciences complex. On top of boasting an interactive science museum, open-air oceanographic park, opera house, and cultural center, it's a true architectural marvel of Spain. It's a massive complex that sits on 350,000 square meters of land and whose pseudo-futuristic structures are all joined by bodies of water.
What to Do
Road trips down the Costa del Sol, food tours of the Basque country, strolling waterfront promenades and visiting world-class museums—these are all ideal activities. For a bit of nature and adventure, head to the south of Andalusia and the town of Tarifa. The offshore winds here make for perfect kitesurfing conditions, and indeed this is destination number one in Spain for this activity. You can also take part in whale-watching excursions from Tarifa to spot sperm whales on their migratory route.
If you happen to be in Spain in mid-September towards the end of the month, then be sure to visit Catalonia. There are festivals during this time that are epic, can't-miss celebrations (more on this below).
Events in September
La Mercé. This festival, which takes place over three days at the end of each September, is the biggest party in Catalonia. Historically the festivities are in remembrance of the Patron Saint of Barcelona, the Virgin Mary, and a plague she supposedly cured in the 17th century. Come to Barcelona during the party and, along with hundreds of thousands of other revelers, you can enjoy fireworks, concerts, parades, processions, drinking, dancing, eating, and much more.
Festa de Santa Tecla. Another great celebration of Catalan culture, the Santa Tecla Festival takes place in Tarragona and lasts for over a week from the middle or end of the month. Like Mercé, it's ostensibly a religious holiday but has grown to become a big street party with festivities, live music, processions, and gastronomic events.
San Sebastian International Film Festival. Held over a week in late September, this has been one of the premier celebrations of cinema since its debut in 1953.
Traveling to Spain in September? Check out these great itineraries.
Best of Barcelona & Valencia - 9 Days. This nine-day itinerary allows you to experience the eclectic city of Barcelona while the weather is still warm enough to enjoy the beaches. It also includes a stop in Valencia so you can check out the architectural marvel that is the City of Arts and Sciences.
Best of Andalusia: 14 Days. Perfect for those who like to be on the move, this two-week adventure takes you on a number of stops between Seville to Jerez de la Frontera for the ultimate tour of Spain's Andalusia region—which enjoys great weather in September.