Nippon/Nihon

What is the official pronounciation of Japan in Japanese? Is it Nihon or Nippon? Thanks a lot!

The official name of Japan is 日本国 (Nihonkoku), so the former is correct.

Thanks a bunch!

So, I'll use Nihon then, as it is indeed the official name and is used by most people in Japan. Thanks!

supplemental

The words Nihon and Nippon are written the same in kanji, they are combined from the kanji NICHI (sun) and HON (origin), hence the name "Land of the rising sun". You can find a nice explanation here: Japan FAQts.
When combining these kanji the combination of these two kanji results in a contraction: NICHI+HON = NIPPON

as a reference: one pencil is counted as ICHI+HON = IPPON

This has been the official name for a long time, however, as happens in all languages, the word has evolved into NIHON, which is now the official name.

NIPPON is still used, but has an archaic ring to it nowadays.