A second-half Mile Jedinak penalty was enough to give Palace a fifth-straight Premier League victory at the expense of West Ham to mathematically seal their spot in the top flight next season.
The Aussie fired home the spot-kick after Cameron Jerome had been felled in the box by Pablo Armero as Tony Pulis' side deservedly picked up another three points to continue their remarkable form at the end of this season.
Palace's penultimate London derby of the season began as textbook as they come with lots of strong challenges and passion but little quality, and nine minutes in a loose clearance by Joel Ward landed at Kevin Nolan's feet and he crossed for the head of Andy Carroll who couldn't keep his header down.
The visitors waited patiently for their first opportunity and came close after a quarter of an hour when Yannick Bolasie's corner which was met by a downwards header from Kagisho Dikgaoi which had to be scrambled off the goalline by Carroll, and soon after the South African stung the palms of Adrian from 20 yards.
Chances were few and far between in the opening half hour but Mohamed Diame tested his luck when he received the ball 30 yards from goal and strolled forward before switching the ball onto his right foot and letting fly, forcing Julian Speroni to tip the ball over his crossbar.
The Argentine was called into action twice in quick succession 10 minutes before the break when he firstly blocked another Diame effort at his near post after he was slotted in by Nolan, and then seconds later he had to swat away a powerful Carroll header with the palm of his hand to ensure a scrappy half ended goalless.
The Hammers continued to enjoy the better of the game when play resumed and Carroll looked desperate to score as he continued to shoot every time he found himself in possession, with his best opportunity coming when Stewart Downing's cross evaded everyone in the area except for the England hitman but he shanked his attempt well wide.
However just before the hour mark it was the Eagles who hit the hosts with a sucker punch when they were awarded a penalty. Jerome sped down the right before trying to turn back inside but saw his legs swiped away Armero, giving Martin Atkinson no option, and skipper Jedinak stepped up to emphatically dispatch the ball into the net from 12 yards and finally get his first Premier League goal.
They nearly doubled their lead after a scramble in the penalty area saw Palace presented with four chances to score in 15 seconds. Jason Puncheon saw a shot blocked before Joe Ledley scuffed the rebound across the face of goal, and then after a saving tackle by two West Ham defenders thwarted Bolasie Puncheon saw another blast deflect wide for a corner.
Bolasie was having another storming game for the Eagles and continued his quest to find his way onto the scoresheet this season but saw another promising chance blocked by George McCarthy as he pulled the trigger, but substitute Joe Cole nearly pulled his side level late on when he pulled a shot wide of the target before blasting a volley high into the stand with five minutes remaining.
However despite a late rally from Sam Allardyce's side the Eagles defence held firm once again to register their fourth clean sheet from their last five outings to secure another well-deserved victory.
West Ham United: Adrian, Reid, McCartney, Nolan (Nocerino 85), Tomkins, Jarvis (C Cole 70), Armero, Carroll, Noble, Diame, Downing (J Cole 78). Subs not used: Taylor, O'Brien, Jaaskelainen, Johnson.
Crystal Palace: Speroni, Mariappa, Dann, Delaney, Ward, Dikgacoi, Jedinak, Puncheon (Gabbidon 87), Ledley, Bolasie (Parr 90), Jerome (Murray 81). Subs not used: Hennessey, Guedioura, Ince, Gayle.