The new restrictions covered six times more trade than in the previous period and were the largest since the WTO started monitoring G20 trade in 2012, it said in a statement.
"The report's findings should be of serious concern for G20 governments and the whole international community," WTO Director-General Roberto Azevedo said in the statement.
"Further escalation remains a real threat. If we continue along the current course, the economic risks will increase, with potential effects for growth, jobs and consumer prices around the world."
The WTO was doing all it could to help de-escalate the situation, he added, but solutions would need political will and leadership from the G20, whose leaders will meet in Argentina next week.
The monthly number of trade restrictions averaged eight during the period covered by the report, up from six per month in the previous report, which covered mid-October 2017 to mid-May 2018, the WTO statement said.
"The proliferation of trade-restrictive actions and the uncertainty created by such actions could place economic recovery in jeopardy. Further escalation would carry potentially large risks for global trade, with knock-on effects for economic growth, jobs and consumer prices around the world," it said.
G20 countries had also implemented a monthly average of almost seven trade-liberalizing measures such as reducing import tariffs and export duties, covering a total of $216 billion of trade, in line with the trend since 2012.
