The current holder of the race leader's yellow jersey was climbing to the finish line at La Toussuire after close to 140km of punishing racing when a spectator leaned in towards the Briton from the roadside and delivered a saliva-soaked insult.
The unsavoury incident comes just a few days after a cup of urine was thrown at the 2013 champion during Stage 14 of the race.
A small minority of race goers have targeted Team Sky through the past weeks, with Froome’s Australian team-mate, Richie Porte, punched in the ribs during Tuesday's stage and Luke Rowe also copping flack.
Froome blamed "irresponsible reporting" from some media companies as the fuel for the hatred, but luckily was unaware of last night's sour note.
“Fortunately for me I didn’t see it but a few journalists have brought that to my attention,” Froome said of the spitting.
“That is appalling behaviour. You can’t come to a bike race and punch riders, or throw urine at them or spit at them. That’s not acceptable.”
Rowe put out something of a seek-and-destroy message on Twitter in an effort to put an end to what has been relentless animosity towards his team.
Even without this ugly incident, it was a rough day for Sky.
Froome lost almost 30 seconds to his nearest general classification rival Nairo Quintana (Movistar) as 2014 champion Vincenzo Nibali (Astana) won the stage after attacking the Briton as he suffered a small mechanical issue.
Froome was grateful for team-mate Wout Poels, who was his only real support across the final 70km of the stage as the rest of Sky’s men fell away.
Geraint Thomas suffered particularly badly and slipped from podium contention into 15th on the GC list.
“I was just empty today,” the Welshman said afterwards.
“Sometimes you’re the hammer, and sometimes you’re the nail. I was a cheapy, horrible little Ikea nail today.”
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Froome remains confident of holding on to his lead with one serious stage remaining in the race, but he is more than aware of Quintana’s ability to spoil any Parisienne party plans.
“It’s going to be epic out there,” Froome said of Stage 20, which sees the riders finish atop the iconic Alpe d’Huez climb.
“It would be a dream to take the stage win, but I’ll have to keep up with a little Colombian first.”
