The corner of the end zone beckoned for Rams running back Darrell Henderson, who took a third-quarter handoff, rounded the edge and sprinted toward the pylon. Scurrying away from Seattle’s line, Henderson evaded one defender — but not the man who raced clear across the formation to drag him down from behind, 2 yards short of the goal line.

“There was no way I was going to let him walk into that end zone,” said Jamal Adams, a Seattle safety.

By preventing a touchdown, Adams preserved the Seahawks’ lead and embodied a smothering defensive effort that fueled their fifth victory in six games, 20-9, over Los Angeles, clinching their first N.F.C. West title since 2016.

Back then, Seattle’s identity revolved around its defense, around a fierce pass rush, formidable secondary and the colorful personalities who powered both. This group doesn’t surpass the lofty standard set by the Legion of Boom. But over the last five weeks, as the playoffs draw near, no team has allowed fewer points (61) than the Seahawks.

“There were times during the season where everybody had enough statistics to go ahead and blow us out, that we weren’t worth anything on defense,” Coach Pete Carroll said. “This defense is good. And they’ve shown it and they’ve declared it. This is the kind of defense that we’ve played in years past.”

Even though the Rams ran more plays and held the ball six minutes longer than Seattle did, they managed only nine points, on three field goals, stifled in moments big and small. The Seahawks (11-4) sacked Jared Goff three times and intercepted him once, and were at their mightiest near their own end zone, stuffing the Rams on four chances inside the Seattle 4-yard line after Adams’s tackle.

Right before that stand, the Rams (9-6), trailing by 13-6, faced second-and-5 from the 7. Adams tore into the backfield as soon as the ball was snapped, pursued Henderson as if powered by rocket fuel and then yanked him down shy of what could have been the tying touchdown. The next four plays went for minus-2, 3, 0 and 0 yards, and after the Seahawks knocked back Malcolm Brown on fourth down, they galloped away together.

“Give us a blade of grass, and we’ll defend it,” said Carroll, who added that he’ll remember that goal-line sequence forever. “We were in full-on attack mode.”

That attack mode did not materialize on offense to the extent it did earlier in the season, when Russell Wilson was flinging touchdown passes at will. But after a desultory first half, Wilson led Seattle on an 80-yard scoring drive, then secured victory with a 13-yard pass to Jacob Hollister with 2 minutes, 51 seconds remaining.

In clinching the division title, Seattle assured itself of earning at least the No. 3 seed in the N.F.C. It also denied the Rams of a playoff berth that seemed certain eight days ago, when they prepared to host the winless Jets.

Even in this week-to-week league, the Rams are as trustworthy as an email from a Nigerian prince. Just as they were approaching stability, winning four of five in a stretch that began with a Week 10 victory against Seattle, they collapsed against Jets. Put another way, after beating Bill Belichick on a short week, Sean McVay had 10 days to out-coach Adam Gase and could not.

Against teams with winning records — teams like the Seahawks — the Rams had won their last four until Sunday.

The stands at Lumen Field, one of the N.F.L.’s rowdier venues, normally would have been shaking for such a critical game, heaving with boisterous fans. The Seahawks yearn for that ambience. So does McVay, who last week said how much he had expected to miss being yelled at. “I kind of like it in a messed-up way,” he said.

Maybe so, but McVay would have preferred seeing evidence that last week’s defeat was an aberration instead of, perhaps, the start of an unsettling trend.

The first half unfolded as an extension of both teams’ recent fortunes, all middling offense and suffocating defense with a modicum of scoring. Had the field been shortened to 60 yards, Seattle and Los Angeles might not have noticed: Neither ran a play in the red zone.

The Rams’ forays into Seahawks territory produced two field goals and an interception that defied justification. On first-and-10 from the 29-yard line, Goff, flushed right, tottered toward the sideline, where a sliver of open space welcomed him. Instead of running, he floated a pass across his body into an area the size of a city park but absent any receivers.

One of a few Seattle defenders nearby, Quandre Diggs swooped in for the interception, Goff’s 13th of the season, and Seattle converted the turnover into the field goal that sent the game sputtering into halftime at 6-6.

When asked what he saw on the interception, Diggs said: “I couldn’t tell you. I mean, my coaches always told me in high school when I was a quarterback, never throw the ball across your body.”

With their stingy defense and excellent coaching, the Rams need not brilliance from Goff, just competence. He does not have to be the reason the Rams win, just not a reason they lose. In a league loaded with dazzling quarterbacks, the term game manager seems to wield a pejorative connotation, but it shouldn’t.

Performing elemental tasks of the position, like making smart decisions quickly and not turning the ball over, is something Goff has done before. But when he fails, it makes Wilson’s capability ever more noticeable.

Reliable quarterback play separated Seattle from the Rams on Sunday. But so did something else: Its defense, once maligned but now, just when the Seahawks needed it, was an asset.