Can I legally marry my fiance to get him his citizenship?

It is important that you get married for the right reasons, if you and your fiance decided to get married that's great, but please hire a lawyer before you submit anything to immigration. When you say he is illegal do you mean he entered without inspection or he overstayed his authorized status? It is a big difference. Again I suggest hiring a lawyer to help you. Good Luck!

When a U.S. citizen married a foreign national, the US Citizen may petition for permanent residence for the foreign national spouse. However, if the spouse entered the U.S. without documents, then the spouse may not be eligible to gain his or permanent residence in the U.S. Only those applicants who were beneficiaries of an immigrant petition or an employer filing prior to April 30, 2001 and who were present in the U.S. in December 2000, are eligible to pay a fine of $1000 and adjust status in the U.S. Those who are ineligible to adjust status in the U.S. because of an illegal entry may be able to return to their home countires to apply for an Immigrant Visa. However, the spouse may need a waiver of the time they have been in the U.S. without status. Not all waivers are approved. Because your case is complicated, you should seek a consultation with a qualified immigration attorney before making any plans. above.