It kept its assumptions for other commodities unchanged, including a gold price forecast of US$1,250 an ounce for the rest of 2018.
The company had boosted its outlook for copper in July, lifting its forecast by $200 per tonne to $6,600/t for the remainder of the year, citing favourable demand fundamentals.
However in a report released this week, it lowered this to $6,000 for the rest of the year and reduced its copper price forecast for the next two years, slashing its 2020 assumption of $7,000/t to $6,200/t.
The copper cash price on the London Metal Exchange has averaged $6,707.51/t over the past year and closed overnight at $5,843.75.
"In our view, the weaker sentiment for copper reflects a rising interest rate environment, relative strength of the US dollar and trade tensions (and risks to future growth) have also tempered our outlook for prices," the report stated.
However S&P said it viewed copper market fundamentals as "fairly healthy", pointing to limited output growth, expecting demand to modestly exceed supply over the next few years but saying long-term demand was less certain given potential disruptions from new technologies.
Credit analyst Jarrett Bilous said periods of pricing volatility in the metals and mining sector was to be expected.
"However, the recent decline for certain commodities had been pronounced and, in our view, more likely to be sustainable," he said.
"The weaker market sentiment for copper, and our expectation for higher zinc supply, underpinned the respective revisions to our assumptions."
S&P lowered its zinc forecast for this year and next, from an assumed $3200/t for the rest of 2018 to $2,800/t, and from $3,000/t to $2,800/t in 2019.
It said the revision reflected its expectation for higher supply amid "relatively tepid" demand growth and said global inventories had expanded after falling for more than two years.
"Moreover, we believe zinc prices at $2,800/t are close to the marginal cost of production for about 75% of zinc producers globally," S&P said.
"While we believe a high degree of price volatility will persist over the next few years, prices close to this level will likely act as a natural balance for demand and supply."
The LME zinc cash price has averaged $3,105.38/t over the past 12 months and last traded at $2,301.25.
S&P said it expected generally favourable commodity demand, primarily linked to its GDP growth forecasts for the US, the Eurozone, Latin America and China.
However risks were building for increased future price volatility in relation to trade tensions and concerns around higher interest rates, it said.