The improvement in outlook of RIL, TCS, ONGC, NTPC, NHPC and PGCIL follows S&P's revision in the sovereign credit outlook for India, S&P's said in a statement.
Earlier, the international rating firm had revised back India's outlook to stable from negative. A stable outlook means that the agency does not see a chance of more than one in three to lower the current credit rating.
S&P's said it revised the outlook on Reliance to reflect the reduced likelihood that it may lower the transfer and convertibility assessment following the sovereign action.
"We expect the government to maintain its majority ownership, and influence the business and financial strategy of the companies," it said.
The outlook on the foreign currency rating on TCS has been revised following similar action on the sovereign, S&P's added.
Further, S&P's also revised the outlooks on four Indian government-related entities -- EXIM Bank, India Infrastructure Finance Company, Indian Railways Finance Corp, Power Finance Corp-- to stable from negative.