The figure represents an increase of $3.27 billion from a month earlier, according to the Bank of Korea (BOK).
It beat the previous record of $328,91 billion last January, the BOK said.
The foreign exchange reserves reached $328.80 billion in April, $328.10 billion in May and $326.44 billion in June.
Securities took the lion's share of $301.34 billion or 91.4 percent of the total foreign exchange reserves in July, followed by deposits ($17.55 billion or 5.3 percent) and gold ($4.79 billion or 1.5 percent), the central bank said.
By Ayaka Erin Tobari
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