Newcastle have lost six consecutive matches in the Premier League, the latest setback coming in Sunday's 3-1 defeat at home to Tottenham, when some fans boycotted.
With five games remaining, they sit seven points clear of a resurgent Leicester City side - who are 17th - ahead of Saturday's match against Swansea City.
"We have to stay positive and keep believing," Janmaat told the Evening Chronicle.
"It's not good. We have to win the minimum of one game.
"There are five games left and we must win one. I believe we can do it. We can't just say it – we have to do it now.
"Now is the moment. Leicester are winning games so we have to stop talking and go out and do it."
Janmaat, who joined Newcastle from Feyenoord in July, came off the bench in Netherlands' World Cup semi-final loss to Argentina last year, but the 25-year-old reckons the upcoming clash with Swansea could rival that occasion.
"It's a big game," he said. "I have played in the semi-finals of the World Cup, that was big but this is close for me.
"We have to win it. We are staying focused.
"Taking three points will be enough to be safe at the moment. That is our only focus."
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