⚑ The chapter's theological centre, and the verse most often used to tell a Christian what they still are.
Quick answer
Nobody can say it. The verse asks a question whose answer is silence, and it is asking about self-cleansing specifically, not about whether anyone has ever been made clean.
Read both lines
The form is a rhetorical question and the expected answer is no one, which the original readers would have supplied instantly. Both verbs are first person and both are active: I have made my heart clean, I am pure from my sin. That is the precise claim being ruled out. The verse is not measuring how much sin a person has; it is asking whether anyone can stand up and take credit for their own purity, and the silence that follows is the answer. Notice also where it locates the problem. Not in behavior, which could be adjusted, but in the heart, which in this book is the seat of judgment and desire and is not available for renovation from the outside.
What it does not mean
This verse is not a description of the Christian's standing before God, and it is regularly used as one. Read that way it says a believer is still impure, still under audit, still waiting for a verdict that Scripture says has already been given, and it turns an honest question about self-cleansing into a permanent status report. The verse rules out one specific boast. It does not rule out what God has done.
It also does not license the opposite error. John writes to Christians that "if we say that we have no sin, we deceive ourselves" (1 John 1:8), which is the same honesty as this proverb about lived experience. A finished standing and an unfinished experience are both true, and collapsing either into the other is where the damage happens. Nobody cleans their own heart. Everyone in Christ has been given one, and that is a standing received rather than a condition anyone reached.
From this side of the cross
The gospel does not change this verse's answer. It changes who is doing the cleansing.
The question "who can say I have made my heart clean?" still has exactly one answer, which is nobody, and the New Testament never softens it. What the New Testament announces is a different sentence altogether, in the passive voice, with someone else as the subject: "For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him" (2 Corinthians 5:21). Not cleaner. Not improving. Made the righteousness of God, in Him. And Hebrews says it as a completed act with an unfinished shadow: "by one offering He has perfected forever those who are being sanctified" (10:14).
So a believer reading 20:9 should feel two things at once and neither should cancel the other. I cannot say I made my heart clean, and I do not need to, because the claim was never mine to make. Titus puts the same thing negatively: "not by works of righteousness which we have done, but according to His mercy He saved us" (3:5).
Related: 2 Corinthians 5:21 · Hebrews 10:14 · 1 John 1:8–9 · Titus 3:5 · Proverbs 16:2 · Proverbs 21:2
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.