Can a Muslim plead not guilty if they did it?

It depends, and scholars differ. In many Western courts, "not guilty" is a legal formula that means "prove it", not a sworn statement that you didn't do it, so some scholars allow it. But you must never lie under oath, invent a false story or let an innocent person take the blame. Speak to a lawyer and a trusted imam.

What a "not guilty" plea means legally

In the UK and USA, a plea of not guilty asks the prosecution to prove its case. Lawyers often describe it as a procedural step rather than a factual claim, and defence lawyers may represent people they believe did the act. Because of this, some contemporary scholars say a plain plea isn't a lie in itself. Others say it still sounds like a denial, so a Muslim who knows they're guilty should avoid it or plead guilty. Both views exist; neither should be stated with total certainty.

Islam doesn't require public confession

Islam encourages people to keep their own private sins hidden and repent to Allah. The Prophet ﷺ said that all of his followers would be pardoned except those who expose their own sins (Sahih al-Bukhari 6069). When a man named Ma'iz came to confess a serious sin, the Prophet ﷺ tried to turn him away, asking whether he had perhaps only kissed or looked (Sahih al-Bukhari 6824). So you aren't religiously obliged to hand yourself in for every wrong. Repentance between you and Allah is valid.

The clear red lines

No false oaths. Swearing falsely in court is a major sin (Sahih al-Bukhari 6675). See Can Muslims swear on the Quran in court?
No invented evidence or false alibis. Fabricating a story is lying, not a defence.
Never blame someone innocent.

وَمَن يَكْسِبْ خَطِيٓـَٔةً أَوْ إِثْمًا ثُمَّ يَرْمِ بِهِۦ بَرِيٓـًٔا فَقَدِ ٱحْتَمَلَ بُهْتَـٰنًا وَإِثْمًا مُّبِينًا "But whoever earns an offense or a sin and then blames it on an innocent [person] has taken upon himself a slander and manifest sin." Quran 4:112

Victims still have rights

If your act harmed a person, such as theft or injury, repentance isn't complete until you return what you took or make it right with them. Pleading guilty can sometimes be the cleanest way to do that, and courts often reduce sentences for an early guilty plea. Your lawyer can explain your options honestly. Muslim lawyers work within this system too. See Can Muslims be lawyers or judges in a secular court?

A fresh start

Whatever you decide, Allah's door is open. A past offence doesn't cancel your Islam or your chance of a new life. See Can I still become Muslim if I have a criminal record?

Key points

Scholars differ on a formal "not guilty" plea when you did it.
False oaths, fake alibis and blaming others are clearly haram.
You aren't obliged to expose private sins; repent to Allah.
Wrongs against people must be put right with them.

Tip: Tell your lawyer the full truth in private; that conversation is confidential. They can then advise you without you ever having to say anything false in court.