The monument has been erected to honour rangers who risk their lives for the sake of conservation.
She said: "As government we need to boost the morale of rangers by showing them that their battle against poachers and other environmental crimes are not in vain.
The event marked the seventh anniversary of World Ranger Day held in memory of rangers who have lost their lives in the line of duty.
It also recognises those who continue to risk their lives daily to protect endangered species and other natural resources.
Thomson said: ''Nowhere is the importance of rangers more clearly illuminated than in the fight against rhino poaching. It goes much deeper than mere physical security.
"Social and economic problems such as unemployment and poverty are part of the problem," Thomson added.
The 'Ranger Monument' includes parts of the Leadwood tree and is meant to symbolise longevity and the ability to overcome challenges.
The unveiling ceremony was held as the number of rhino poached in South Africa since 1 January 2014 rose to 618 and 400 of those were killed in the park.