The knockout stage was the second and final stage of the 2018 AFF Championship, following the group stage. It was played from 1 to 15 December with the top two teams from each group (two in total) advancing to the knockout stage to compete in a single-elimination tournament. Each tie was played on a home-and-away two-legged basis. The away goals rule, extra time and penalty shoot-out were used to decide the winner if necessary.

Vietnam won 3–2 on aggregate against Malaysia in the final to win their second title.

The top two placed teams from each of the two groups advanced to the knockout stage. In Group A, Vietnam secured the group top spots with 10 points after defeating Cambodia by 3–0 in their last match while Malaysia became the group runners-up with 9 points after beating Myanmar with similar scores of 3–0.^[1][2] Meanwhile in Group B, Thailand secured the group top spots with 10 points after defeating Singapore by 3–0 in their last match while Philippines became the group runners-up with 8 points after drawing 0–0 against Indonesia.^[3][4]

More information Group, Winners ...

Group	Winners	Runners-up
A	Vietnam	Malaysia
B	Thailand	Philippines

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The schedule of each round was as follows.

	Semi-finals							Final
	A2	Malaysia (a)	0	2	2
	B1	Thailand	0	2	2
								A2	Malaysia	2	0	2
								A1	Vietnam	2	1	3
	B2	Philippines	1	1	2
	A1	Vietnam	2	2	4

Scores after extra time are indicated by (a.e.t.), and penalty shoot-out are indicated by (pen.).