An injury-time Murray Wallace strike secured a 3-2 victory over Everton in a controversial rain-soaked Den encounter.
With the tie heading towards a replay after Jake Cooper’s 75th minute equaliser appeared to come off his arm, Wallace slotted home to send the home fans into raptures.
The victory means Millwall have now won five FA Cup games in a row against Premier League opposition going into Monday night’s Fifth Round draw.
In a low-key first-half, Richarlison’s low 25-yard effort squirmed under Millwall stopper Jordan Archer to give Everton a 43rd minute lead.
But that lasted only four minutes as Lee Gregory’s looping header over Jordan Pickford saw the Lions level at the interval.
As conditions worsened in the second period, Gylfi Sigurdsson expertly played in substitute Cenk Tosun to slot home for Everton’s second (72).
But once again Millwall replied immediately – Cooper getting on the end of Wallace’s pass to make it 2-2 just three minutes later.
And as the legitimate claims for handball were waved away, Wallace further compounded Everton’s misery with his last-gasp stoppage time winner.