Peru has three very different regions, each with its own climate logic. The best time to visit depends on where you are going.
The Andes and Machu Picchu
The dry season runs from May to October. Skies are clear, trails are dry, and conditions for hiking the Inca Trail or exploring the Sacred Valley are at their best. June, July, and August are peak months — book accommodation and Inca Trail permits well ahead. Nights can be cold at altitude even in the dry season; Cusco sits at 3,400m.
The wet season (November to April) brings afternoon rain and occasionally closes trekking routes. January and February see the heaviest rainfall. The Inca Trail closes entirely during February for maintenance. That said, the wet season means far fewer crowds, cheaper prices, and landscapes that are brilliantly green.
The Coast
The coast around Lima is overcast and mild from June to November — locals call this the garua season, a low cloud that rarely produces rain but blocks the sun. December to April is warmer, sunnier, and considered the coast's summer. Beach towns like Mancora in the far north are warm year-round.
The Amazon
The Amazon basin is hot and humid throughout the year. The "high water" season (December to May) floods river banks and makes some areas easier to explore by boat; the "low water" season (June to November) reveals beaches and concentrates wildlife around remaining water sources. Both have their advantages for wildlife-watching.
In Summary
For a first visit combining the highlands and coast, May to September is the safe choice. For budget travelers and those who don't mind weather, the shoulder months of April and October offer a good compromise of conditions and crowds.