Strong investor appetite for bonds across the curve has kept spreads grinding tighter: Single A rated bonds also hit a new post-crisis tight of T+75bp, the data show.
"We're picking up from where we left off in a number of risk markets, particularly investment grade," said Todd Schomberg, senior portfolio manager at Invesco.
"Corporate tax reform, including repatriation of cash and limits on interest deductibility, means the market has a rosy outlook on IG credit."
Borrowers have seized the chance to sell new debt in the usual busy period for issuance at the start of the year.
Investment-grade borrowers have raised US$36.6bn so far this year, according to IFR data, and another seven issuers are expected to price deals Tuesday.
The junk-bond market is also off to a strong start in 2018.
The average US high-yield bond spread over Treasuries was at 335bp on Monday, which matches the post-crisis low set in June 2014, according to ICE BAML data.
Spreads in the high-yield Auto, Bank, Basic Industry and Industrials sectors also all hit three-year tights Monday.
The high-yield primary saw its first deal of the year price Monday as retailer L Brands - best known for its Victoria's Secret lingerie brand - raised US$500m from a 10-year bond to refinance more expensive debt paying interest at 8.5pc.
The new deal, which priced at the tight end of guidance, will pay interest of just 5.25pc.
Petroleum distributor Sunoco is expected to sell a US$1.75bn three-part deal on Tuesday, and acquisition finance bonds for media conglomerate Meredith and Arby's Restaurant Group are among deals in the pipeline.