Map from Google - Phi_Phi_Islands

The climate of the Phi Phi islands, an archipelago in the Andaman Sea belonging to the province of Krabi, is tropical, hot all year round, with a rainy season, due to the southwest monsoon, from May to October, and a dry season from December to March. In April, before the monsoon, there is a certain rise in rainshowers, while in November there may still be abundant rainfall, especially in the first part, either for any delay in the retreat of the monsoon, or by the passage of a tropical cyclone (see below). Even in the dry season, however, there may be some showers or downpours, especially in December, when occasionally one among them can be particularly intense.
The archipelago consists of the main islands, Phi Phi Don and Phi Phi Leh, and some smaller islands such as Ko Mai Phai (also called Bamboo Island), Bida Nok and Bida Nai. They are coral islands, where tourists practice scuba diving. The islands have been hard-hit by the tsunami in December 2004, and the infrastructures have been rebuilt in the following years.

Map of Phi Phi islands and surrounding areas

Temperatures are high throughout the year, so that average highs never drop below 30 °C (86 °F). However, from December to March the humidity is slightly lower. Night temperatures very rarely drop below 20 °C (68 °F), although this may happen sometimes between December and March. From February to April, before the arrival of the monsoon, there is a slight rise in temperature, so March and April (along with May, the first monsoonal month) are usually the hottest months of the year, in which highs can quite often reach 35/36 °C (95/97 °F).
Here are the average temperatures of the main island, Phi Phi Don.
Average temperatures - Phi Phi Don
Phi Phi DonJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)222223242424242424232323
Max (°C)313333333332313131313031
Min (°F)727273757575757575737373
Max (°F)889191919190888888888688

In the Phi Phi islands, an average of approximately 2,400 millimetres (94.5 inches) of rain falls each year; in these islands and in the Andaman Sea coast, the rains during the period of the southwest monsoon are more abundant than in the other coast of peninsular Thailand, that of the Gulf of Siam, because the south-western winds affect them directly. However, the rainy season sees two peaks, in the first part (in May) and the last (September-October). In some years, however, there can be torrential rains even in July and August.
Here is the average precipitation.
Average precipitation - Phi Phi Don
Phi Phi DonJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)502550130310260275260375330230902380

The amount of sunshine in the Phi Phi islands is good in the dry season, from December to March, while during the rainy season the sun is not lacking, but it alternates with cloudiness, which can sometimes give rise to rain showers: there are on average 9 hours of sunshine per day from January to March, 5 in September, and 6 in the other months from May to November.

From May to November, the Phi Phi islands like the rest of Thailand may be affected by tropical cyclones, although this area is relatively protected, since cyclones develop in two areas, one to the west (the Bay of Bengal, where they are called cyclones), and one to the east (the Pacific Ocean and the South China Sea, where they are known as typhoons); sometimes however some of them may pass even here.

The Beach, Maya Bay, Phi Phi Leh island

For swimming, the sea in the Phi Phi island is warm all year round, as you can see in the following table.
Sea temperature - Phi Phi Don
Phi Phi DonJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)282829303030292929292928
Sea (°F)828284868686848484848482

When to go

The best time to go to the Phi Phi islands and enjoy their sun and sea goes from December to March, being the dry season. As mentioned, in December and early January at times there may still be some showers, while in February and March it can be hot.