France's Foreign Ministry has called it a "violation of international law”.
The delegation, led by Thierry Mariani of the centre-right Les Républicains party, arrived in Crimea on Thursday after earlier meeting with Russian MPs in Moscow.
On Friday they will visit the peninsula's regional capital Simferopol, the port of Sevastopol and the resort city of Yalta.
"It is with deep concern and indignation that I learned of your decision to visit, as head of the French delegation, the Autonomous Republic of Crimea which is temporarily occupied by Russia," Ukrainian ambassador Oleg Shamshur wrote to Mariani in a letter published Friday.
"You did not even inform the Ukrainian side, deciding to enter illegally from Moscow," he said, warning the MPs against the information they receive during the visit.
The tour will be, he said, "fully organised and controlled by the Russian authorities. You will be used to implement the Kremlin’s attempts to legitimise the Crimea annexation."
Crimea was annexed by Moscow last year after a disputed referendum, which most of the world has considered illegal. The European Union has since imposed sanctions that include visa bans, frozen assets and blocked investments in or imports from Crimea. That includes tourism on the peninsula.
Sergei Aksyonov, the pro-Russian leader in Crimea, described the French visit as "recognition by foreign MPs that the sanctions against Russia are ineffective".
French Foreign Minister Laurent Fabius said he was "shocked" by the visit.
But Mariani told French news agency AFP that the MPs are free to go where they wish.
"The first lesson of democracy is the separation of powers. We do not represent neither the French government nor the Assembly as a whole. We have fulfilled all prior obligations," he said.