Spain -weather in mid to late May

There is no average temp for Spain that is accurate all over. It's a big place, with several different climates. What area are you looking at? I have been to Spain three times in recent years, from late April through late May. I have mostly been in the Madrid area and to the north. Last May, it was warm enough to go without a jacket most of the week, when I was around Madrid, and a little cooler and windier on the northern coast (though one day it was warm enough to wade in the ocean). It threatened rain a couple of days, too. In a previous year, it was 104F in Sevilla on May 4, when I was there. You just can't tell, based on averages, what is likely to happen. Late May, though, is generally pleasant weather.

I think it would be perfect for you at that time. My guideline for best times re weather in Spain ...
Barcelona, Valencia, Madrid - May to Oct (with heat in July/Aug)
Basque, Cantabria, Asturias, Galicia - late June to Sep
Andalusia - April to early Nov (with extreme heat in July/Aug)

And for those on here who complain about extreme heat in parts of Spain during mid summer, look at this forecast for a town in outback Australia this week ... 45, 44, 47, 40, 39, 41, 42 celsius
http://www.bom.gov.au/nsw/forecasts/bourke.shtml

That is a little like asking about the average of Colorado. It depends on where you are at. Could be -30 (-20F) in the mountains and 16 (60) on the front range for average of -6 (20) ??? Spain has mountains and low country sooooooo where are you going to be?

I plan to travel from Barcelona, Ibiza, down to Granada and possibly Seville. I guess I'm more concerned about Ibiza to be honest.

Average temperatures don't tell you an awful lot. I look at them, and if I'm still interested in the destination at that time of year, I go to wundergound.com and find the day-by-day historical temps for several recent years. That will give you an idea of the range of temps you can expect.

Enter the destination.
Choose "History"
Enter a date in a prior year within the period you expect to travel
Click "View"
If the display doesn't default to the "Monthly" tab, click on it.
Scroll down to the line graph showing temperature. It shows temps throughout each day.

We were there in early May a few years ago and it was quite chilly. We had packed for warmer weather -- I was grateful to have kept my silks in the suitcase.

Thanks everyone...this was great information!

Ibiza is easy - just look at when the clubs open up for the summer.

To me weather includes things other than just temperature. We were in Spain last May and it rained off and on our first 9 days through Madrid, Toledo, Cordoba, and Granada.

I started out in Madrid on May 16, 2016, and the first rain I remember occurred on June 3 in Plasencia, but it wasn't heavy. Once I got up into northern Spain on June 10, I encountered some rain at least every few days. It's wet up there.

There's a lot of luck involved. Prior to 2015 I'd have said my European-weather luck was quite poor. After two good trips (though 2015 was terribly hot), I fear I'm due for trouble in France and Great Britain this year.

Weather is dependent on where you'll be. Extremadura will be different from Galicia or Almeria. If you wait and go too much later you run into the major tourist season where hotel prices rise and there are more crowds.