An armrest assembly comprising: a support column; and a mounting plate disposed on said support column, characterized in that: said mounting plate has a first moving slot which is elongated in a first direction; said armrest assembly further comprising: a pivotal unit including first and second pivot shafts (41, 42) which are aligned with each other in a second direction that is transverse to the first direction, said first pivot shaft extending through said first moving slot and movable relative to said mounting plate in the first direction, said second pivot shaft being disposed on said mounting plate; an armrest plate having a sliding slot which is elongated in the second direction and through which said first and second pivot shafts (41, 42) extend to be sliable in the second direction so as to permit said armrest plate to be movable relative to said mounting plate in the second direction and to be turnable about said second pivot shaft; and a retaining unit including at least one retaining module which is mounted on said pivotal unit, and a retaining plate which is disposed adjacent to said retaining module and which is configured to cooperate with said retaining module to produce a magnetically attractive force that generates resistance against a relative movement between said retaining module and said retaining plate to resist a movement of said armrest plate relative to said mounting plate. a support column; and a mounting plate disposed on said support column, characterized in that: said mounting plate has a first moving slot which is elongated in a first direction; said armrest assembly further comprising: a pivotal unit including first and second pivot shafts (41, 42) which are aligned with each other in a second direction that is transverse to the first direction, said first pivot shaft extending through said first moving slot and movable relative to said mounting plate in the first direction, said second pivot shaft being disposed on said mounting plate; an armrest plate having a sliding slot which is elongated in the second direction and through which said first and second pivot shafts (41, 42) extend to be sliable in the second direction so as to permit said armrest plate to be movable relative to said mounting plate in the second direction and to be turnable about said second pivot shaft; and a retaining unit including at least one retaining module which is mounted on said pivotal unit, and a retaining plate which is disposed adjacent to said retaining module and which is configured to cooperate with said retaining module to produce a magnetically attractive force that generates resistance against a relative movement between said retaining module and said retaining plate to resist a movement of said armrest plate relative to said mounting plate. said mounting plate has a first moving slot which is elongated in a first direction; said armrest assembly further comprising: a pivotal unit including first and second pivot shafts (41, 42) which are aligned with each other in a second direction that is transverse to the first direction, said first pivot shaft extending through said first moving slot and movable relative to said mounting plate in the first direction, said second pivot shaft being disposed on said mounting plate; an armrest plate having a sliding slot which is elongated in the second direction and through which said first and second pivot shafts (41, 42) extend to be sliable in the second direction so as to permit said armrest plate to be movable relative to said mounting plate in the second direction and to be turnable about said second pivot shaft; and a retaining unit including at least one retaining module which is mounted on said pivotal unit, and a retaining plate which is disposed adjacent to said retaining module and which is configured to cooperate with said retaining module to produce a magnetically attractive force that generates resistance against a relative movement between said retaining module and said retaining plate to resist a movement of said armrest plate relative to said mounting plate. a pivotal unit including first and second pivot shafts (41, 42) which are aligned with each other in a second direction that is transverse to the first direction, said first pivot shaft extending through said first moving slot and movable relative to said mounting plate in the first direction, said second pivot shaft being disposed on said mounting plate; an armrest plate having a sliding slot which is elongated in the second direction and through which said first and second pivot shafts (41, 42) extend to be sliable in the second direction so as to permit said armrest plate to be movable relative to said mounting plate in the second direction and to be turnable about said second pivot shaft; and a retaining unit including at least one retaining module which is mounted on said pivotal unit, and a retaining plate which is disposed adjacent to said retaining module and which is configured to cooperate with said retaining module to produce a magnetically attractive force that generates resistance against a relative movement between said retaining module and said retaining plate to resist a movement of said armrest plate relative to said mounting plate.