It lowered its outlook for the red metal for the rest of this year to US$5900 per tonne from $6300/t.
Copper is currently sitting at around $5750/t.
"The US decision to impose tariffs to a large portfolio of Chinese exports into the US has triggered a large sell-off in copper positions that made the price dip deeper," S&P said.
"Prospects for weaker imports of refined copper from China and the yuan depreciation are likely to keep downside pressure for some time."
For next year, it now expects copper to average $6000/t, down from the previous forecast of $6500/t.
S&P's 2021 forecast has been lowered to $6100/t from $6700/t.
"Downside potential for copper prices is probably low, assuming no drastic changes to the global trade flows," it said.
"We expect a mild deficit for concentrate copper in 2019 with chances of becoming ever more pronounced towards 2021 as supply constraints are becoming more frequent.
"Prospects for large greenfield projects such as Quellaveco and Tia Maria in Peru are getting gloomier, due to the social and political opposition to these projects - a recurrent issue in some parts of Peru - a country that supplies nearly 10% of the world's copper supply."
For the same reasons, as well as more supply entering the market, S&P lowered its zinc forecasts by $200 to $2400/t for the rest of 2019, and $2300/t for 2020 and 2021, which is about $100 below where it's sitting now.
While S&P lifted its rest of 2019 forecast for nickel from $12,000/t to $17,000/t, it lifted its 2020 price forecast to just $15,000/t, about $2800/t below the current price.
Nickel has outperformed this year due to a looming Indonesian export ban.
"We currently believe the major bullish impact is likely to be relatively short-term, with the market stabilising through 2020," S&P said.
"The demand stemming from the stainless steel industry, though showing supporting healthy statistics recently, is susceptible to fluctuations driven by trade tensions, and we deem the current level of nickel prices to be unsustainable for the steel producers.
"The stockpiles of nickel, though declining in 2019 (LME stock is down by 50,000t to 156,000t since the beginning of the year), are enough to cover the moderate market deficit in the next couple of years, in our view."
S&P sees gold averaging $1450 an ounce for the rest of the year, below the current price of about $1500/oz.
It has lifted its 2020 and 2021 forecasts by $100/oz to $1400/oz.
"Our assumptions are modestly below prevailing prices; we incorporate the potential for market sentiment to quickly change and lead to short-term volatility," S&P said.
"We also believe prices beyond 2021 will average $1,300/oz, trending toward what we consider a long-term average level.
"The gold price forward curve is basically flat (compared to a modest upward slope in past periods). In addition, S&P currently expects a slight uptick in nominal US interest rates next year - albeit with lower real rates - which we believe should limit sustained appreciation."
Iron ore is expected to average $80/t for the rest of the year and 2020, before dropping to $70/t in 2021.