US President Donald Trump said he authorised a doubling of steel and aluminium tariffs on Turkey, adding to the pressure on the lira amid souring diplomatic relations between Washington and Ankara.
The move could affect $1.7 billion of Turkish exports.
The Turkish lira hit a record low on Friday before Mr Trump's tweet, which also said "our relations with Turkey are not good at this time!"
The timing of Mr Trump's tweet coincided with an announcement by Turkey's Treasury and Finance Minister Berat Albayrak of the government's new economic policy which is aimed at restoring confidence in the lira. Albayrak is the Turkish President Recep Tayyip Erdogan's son-in-law.
The lira weakened 20% to dollar after his tweet. US stock markets also fell after his tweet.
Ties have been strained by differences over Syria, and the trial in Turkey of US evangelical Christian pastor Andrew Brunson for allegedly supporting the group Ankara blames for an attempted coup in 2016. Mr Brunson denies the charge.
The United States had warned it was reviewing Turkey's duty-free access to its markets, after Ankara imposed retaliatory tariffs on US goods in response to American tariffs on steel and aluminium.
Earlier on Friday, Mr Erdogan called on Turks to support the struggling the lira by exchanging any foreign money, saying Turkey faced "an economic war".
"If you have dollars, euros or gold under your pillow, go to banks to exchange them for Turkish lira. It is a national fight," Mr Erdogan said in comments broadcast on national television.
"This will be the response to those who have declared an economic war," blaming Turkey's woes on what he described as an "interest rate lobby" seeking to push the country to higher rates.