Buttler was heard hurling expletives at the Proteas all-rounder on the final day of the tourists' 189-run series-levelling triumph in Cape Town.
The England wicketkeeper-batsman was found guilty of breaching Level 1 of the ICC Code of Conduct.
Buttler was also ruled to have breached Article 2.3 of the Code of Conduct for Players and Player Support Personnel, which relates to “use of an audible obscenity during an international match”.
The third match of the series gets under way in Port Elizabeth next Thursday.
England's wicketkeeper-batsman was fined 15 per cent of his match fee after stump mics picked up his expletive-laden rant at Philander, who appeared to peeve Buttler by not moving when a throw came in from a fielder.
The incident occurred on a tense fifth and final day at Newlands, when the tourists eventually got the wickets they needed to seal a 189-run victory inside the last hour.
Buttler noted the situation was a factor in his comments, but admitted they were unacceptable.
"I'd like to apologise, I understand that's not the way to behave," he told BBC's Test Match Special.
"As role models we have a duty to behave in a certain way.
"In Test cricket there are lots of high emotions at times and things can be said that don't necessarily mean anything, but for viewers at home it can come across very poorly.
"Sometimes it's just one of those things, the heat of the moment and a bit of red mist.
"It's all done so hopefully it can be swept under the carpet and we move on. I'll take the slap on the wrists."
However, Buttler hinted that players would prefer stump mics not to be turned up so such incidents could remain between those out on the field.
"It's a tough one," he added.
"We understand it adds to the viewership experience having stump mics and being able to hear what goes on, but we also like that what goes on the field, stays on the field and isn't necessarily heard by everyone at home.
"I'm sure it's up for debate."
England's win meant the best-of-four-match series is level at 1-1 ahead of the third Test, which begins in Port Elizabeth on Thursday.
An illness bug has hindered the tourists throughout their time in South Africa and captain Joe Root was the latest to miss training on Sunday.
"It's obviously a frustration, nobody will miss being at training more than Joe," Buttler said.
"It's been a bit of a story of the tour so far, guys getting ill, but I'm sure he'll be fine. Hopefully it's just today."
The seamer signed a Kolpak deal with the county after calling time on his South Africa career following the Test series against England earlier this year.
Philander, who turns 35 next month, was due to join Somerset early in April but will not play for the club this season.
There will be no professional cricket in England and Wales until July 1 at the earliest, so Somerset and Philander agreed to cancel the former Proteas bowler's contract with immediate effect.
Somerset director of cricket Andy Hurry said: "These are clearly very uncertain and challenging times for all Counties with no cricket until at least the 1st July and the likelihood of a condensed and truncated season.
"It was such an exciting prospect to visualise Vernon returning to the County Ground, but given the current uncertainty the cancellation of his contract was a prudent and sensible decision.
"I would like to thank Vernon and his management team for their understanding of the current situation and we hope one day to see Vernon back wearing Somerset colours."
The South Africa captain will be 36 by the time of this year's tournament in Australia, which begins in October, and knows he may need to lighten the load.
Team-mate Vernon Philander, 34, is set to retire from international cricket after the ongoing four-Test series with England and that gave Du Plessis reason to reflect.
"I was the last one standing now with Vern [Vernon Philander] going from our generation," he said in a news conference.
"I said before that the T20 World Cup would be a crossroad in my career where I would see where I'm at.
"I think something will have to give from a format point of view. I don't know what this is yet.
"As a team probably if you look at the next year, Test cricket and T20 is probably our two main focus points. So possibly in the one-day game [we need to] start looking at guys who will get opportunities.
"I think it's important we do that more - get guys ready."
Du Plessis has played 62 Test matches, 143 ODIs and 44 T20 internationals ahead of the second five-day match with England, which starts on Friday.
England sealed a 3-1 series win by wrapping up a 191-run victory with a day to spare in the fourth and final Test at the Wanderers.
The match was far from a dream send-off for Philander, who having struggled with a hamstring injury ended with just 14 runs and two wickets as the Proteas lost a third straight contest
"I'd like to thank Vern for his services to the Proteas over the years," Du Plessis said after Philander was given a standing ovation and a guard of honour from his team-mates having been dismissed for 10 by man of the match Mark Wood.
"This team will miss him so, so much. We will sit with him tonight in the dressing room and share in the memories.
"Off the field he's been a champion guy and I'm sad this game wasn't the fairytale moment he deserves.
"It's a lot of effort and time away from family, the sacrifices you make for over 10 years. We're very thankful."
Du Plessis, who has hinted his own Test career may be nearing its end, acknowledged the Proteas had fallen short, particularly when it came close to their batting.
The skipper added: "Right through this series after that first game, England were better in every department than us.
"We did play well in that first game, but one Test doesn't make a summer. We didn't put enough runs on the board, though Rassie van der Dussen played well here and was unlucky not to get a great hundred.
"Anrich Nortje came through shining for us, we need pace to take the place of the Steyns and Morkels, and he has come through - the skill to land the ball in the same area is what has impressed me the most.
"Beuran Hendricks came in with five in his first Test match but from a batting point of view we weren't at our best and the scores reflect that."
Philander was keen to credit England after his Test career came to a close.
"Obviously not the way I wanted to end," he said.
"All credit to Joe Root and his team. We all fight hard on the park but at the end of the day we remain gentlemen.
"To my guys, thanks for making it such an honour for me to wear this badge in the last few years. It's been an honour and privilege to share a dressing room with so many greats.
"I think now the hard work starts, giving back to young talent is my mission in life, hopefully I can mentor a couple to play for this wonderful team one day."
Philander's exchange with Buttler continued a feud between the pair that saw the England wicketkeeper-batsman himself sanctioned for comments made during the second Test.
The Proteas star breached Article 2.5 of the ICC Code of Conduct, relating to "using language, actions or gestures which disparage or which could provoke an aggressive reaction from a batter upon his/her dismissal during an International Match".
On Sunday, Philander accepted a fine and a demerit point for his send-off of Buttler in England's first innings the previous day, although the latter punishment counts for little as Philander prepares to retire.
Philander bowled only nine balls in the tourists' second innings due to a hamstring injury, Joe Root leading his side to 248 all out, setting South Africa a huge target of 466 for victory.
Along with Philander and Buttler, both Kagiso Rabada - consequently suspended - and Ben Stokes have received fines and demerit points in this series, which England lead 2-1 going into the last two days of the fourth and final match.
The tourists had a great opportunity to show a positive response to losing the first Test at Centurion when Joe Root won the toss and opted to bat on a good track in Cape Town on Friday.
It was the Proteas' day, though, as England crumbled abjectly in the sun to close on 262-9, having been 185-4 prior to Ben Stokes playing a poor shot to depart for 47.
Pope was the only batsman to score a half-century on his return from illness a day after his 22nd birthday, making a mature unbeaten 56 with support from last man James Anderson in an unbroken last-wicket stand of 28.
Vernon Philander, Kagiso Rabada, Anrich Nortje and Dwaine Pretorius took two wickets apiece for a South Africa side aiming for a fifth successive win at the famous venue, a place where England have not secured victory for 63 years.
Philander (2-46) wasted no time in making an impact in his last Test on his home ground, Zak Crawley - in for the injured Rory Burns - edging behind in the third over.
Dom Sibley (34) found the rope seven times before he nicked Rabada (2-63) to a diving Quinton de Kock to end a second-wicket stand of 55 with Joe Denly just before lunch.
Denly was rattled on the helmet as Nortje tested him with some short stuff and Root (35) was undone by a hostile delivery from the same bowler, trudging off in fury after gloving the quick to wicketkeeper De Kock for 35 straight after he had been dropped in the slips.
Denly (38) fell seven overs later, Keshav Maharaj breaching his otherwise solid defence to bowl the number three and leave England in a spot of bother on 127-4.
Stokes looked in ominous touch on the ground where he made a career-best 258 four years ago as he and Pope attempted a rebuilding job, including clattering Maharaj into the crowd.
The all-rounder continued to play positively as Pope accumulated steadily, but Stokes gave his wicket away by tamely chipping Nortje (2-54) to Dean Elgar at cover in the final session.
Jos Buttler (29 from 27 balls) played in one-day mode and put Maharaj on the building site with a fierce blow before Pretorius (2-26) ended his promising knock.
Sam Curran lost his off stump offering no shot to Pretorius, then Philander got rid of Dom Bess – the spinner selected to replace the injured Jofra Archer in England's attack - for a golden duck with the first delivery with the second new ball, De Kock taking his fifth catch.
England were 234-9 when Stuart Broad got himself into an almighty tangle and was bowled by Rabada, but Pope ramped the paceman for two fours and farmed the strike impressively to make a second Test half-century.
The right-hander had a life when he was caught hooking Rabada as the paceman overstepped but while the final wicket proved to be elusive for South Africa prior to the close, they will be much the happier of the two sides.
