It's a little tricky because it looks like a feminine noun, but it is, in fact, masculine. This is one of those words that doesn't follow the usual gender rules in French.
Here's how you can remember it:
* Think of the season: "L'été" (the summer) is a male season in French.
* Think of "le temps": "L'été" is part of "le temps" (the weather), which is masculine.
So, when talking about "été", use the masculine definite article "le" before it, or "un" if it's an indefinite article.