Japanese stocks slid on Wednesday in thin trade before Federal Reserve Chair Janet Yellen's comments later in the day, while exporters were hit by a stronger yen in the wake of a fresh controversy for US President Donald Trump's administration.

The Nikkei shed 0.5 per cent to 20,098.38 and the broader Topix dropped 0.5 per cent to 1,619.34.

Trading was subdued, with investors looking ahead to US monetary policy clues from Yellen's testimony to Congress later in the day. The Topix's turnover was 1.98 trillion yen, the lowest since June 26.

“Global investors have adjusted their positions before the event,” said Chihiro Ohta, general manager of investment research at SMBC Nikko Securities.

Exporters, which gained over the past few days on hopes for better earnings thanks to a weaker yen, lost ground on profit-taking.

Advantest Corp dropped 0.6 per cent, TDK Corp declined 1.3 per cent and Toyota Motor Corp lost 0.6 per cent after the dollar slipped 0.4 percent to 113.48 yen, moving away from a four-month high of 114.495 yen on Tuesday.

On Tuesday, Wall Street was roiled briefly after emails disclosed Trump's eldest son welcomed help from a Russian lawyer for his father's 2016 election campaign against Hillary Clinton.

Japanese analysts said the impact from the political turmoil in Washington on the Japanese stock market was limited.

Some Tokyo stocks were in the spotlight on individual news.

Women's wear company Honeys Holdings soared 9.3 per cent and was the seventh biggest gainer on the board after it forecast a 30.5 per cent increase in operating profit for the year through March 2018.

But Sapporo Holdings dropped 3.2 percent after the Nikkei business daily reported that the brewer's January-June operating profit likely was a little bit over 3 billion yen, falling short of about 4 billion yen that the market has expected