CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 29, 2018	$ (192,242)	$ 186,519	$ 1	$ 143,794	$ (101)	$ (335,936)
Balance, Shares at Dec. 29, 2018		50,404,140	1,397,493
Issuance of common stock upon the exercise of common stock options	8			8
Issuance of common stock upon the exercise of common stock options, Shares			29,180
Stock-based compensation expense	57			57
Foreign currency translation adjustment	34				34
Net loss	(6,895)					(6,895)
Balance at Mar. 30, 2019	(199,038)	$ 186,519	$ 1	143,859	(67)	(342,831)
Balance, Shares at Mar. 30, 2019		50,404,140	1,426,673
Conversion of convertible preferred stock into common stock upon initial public offering	186,519		$ 186,519
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		(50,404,140)
Conversion of convertible preferred stock into common stock upon initial public offering		$ (186,519)
Conversion of convertible preferred stock into common stock upon initial public offering, Shares			13,119,424
Conversion of TransMedics' common stock into TransMedics Group's common stock upon corporate reorganization			$ 143,859	$ (143,859)
Conversion of preferred stock warrants into common stock warrants upon initial public offering	1,239		1,239
Issuance of common stock upon the exercise of common stock options	6		$ 6
Issuance of common stock upon the exercise of common stock options, Shares			8,771
Issuance of common stock in initial public offering,net of discounts and issuance costs	91,401		$ 91,401
Issuance of common stock in initial public offering,net of discounts and issuance costs, Shares			6,543,500
Stock-based compensation expense	220		$ 220
Foreign currency translation adjustment	(21)				(21)
Unrealized gains (losses) on marketable securities, net of tax of $0	30				30
Net loss	(9,195)					(9,195)
Balance at Jun. 29, 2019	71,161		$ 423,245		(58)	(352,026)
Balance, Shares at Jun. 29, 2019			21,098,368
Issuance of common stock upon the exercise of common stock options	94		$ 94
Issuance of common stock upon the exercise of common stock options, Shares			63,065
Settlement of accruedfinancing fee	124		$ 124
Stock-based compensation expense	295		295
Foreign currency translation adjustment	56				56
Unrealized gains (losses) on marketable securities, net of tax of $0	22				22
Net loss	(8,280)					(8,280)
Balance at Sep. 28, 2019	63,472		$ 423,758		20	(360,306)
Balance, Shares at Sep. 28, 2019			21,161,433
Balance at Dec. 28, 2019	54,649		$ 424,134		(2)	(369,483)
Balance, Shares at Dec. 28, 2019			21,184,524
Issuance of common stock upon the exercise of common stock options	75		$ 75
Issuance of common stock upon the exercise of common stock options, Shares			146,793
Issuance of common stock in connection with employee stock purchase plan	197		$ 197
Issuance of common stock in connection with employee stock purchase plan , Shares			12,163
Stock-based compensation expense	385		$ 385
Foreign currency translation adjustment	16				16
Unrealized gains (losses) on marketable securities, net of tax of $0	213				213
Net loss	(8,852)					(8,852)
Balance at Mar. 31, 2020	46,683		$ 424,791		227	(378,335)
Balance, Shares at Mar. 31, 2020			21,343,480
Balance at Dec. 28, 2019	54,649		$ 424,134		(2)	(369,483)
Balance, Shares at Dec. 28, 2019			21,184,524
Foreign currency translation adjustment	(10)
Unrealized gains (losses) on marketable securities, net of tax of $0	(4)
Net loss	(22,437)
Balance at Sep. 30, 2020	109,550		$ 501,486		(16)	(391,920)
Balance, Shares at Sep. 30, 2020			27,159,065
Balance at Mar. 31, 2020	46,683		$ 424,791		227	(378,335)
Balance, Shares at Mar. 31, 2020			21,343,480
Issuance of common stock upon the exercise of common stock options	92		$ 92
Issuance of common stock upon the exercise of common stock options, Shares			42,882
Issuance of common stock in initial public offering,net of discounts and issuance costs	75,042		$ 75,042
Issuance of common stock in initial public offering,net of discounts and issuance costs, Shares			5,750,000
Stock-based compensation expense	631		$ 631
Foreign currency translation adjustment	(42)				(42)
Unrealized gains (losses) on marketable securities, net of tax of $0	(149)				(149)
Net loss	(8,497)					(8,497)
Balance at Jun. 30, 2020	113,760		$ 500,556		36	(386,832)
Balance, Shares at Jun. 30, 2020			27,136,362
Issuance of common stock upon the exercise of common stock options	27		$ 27
Issuance of common stock upon the exercise of common stock options, Shares			12,169
Issuance of common stock in connection with employee stock purchase plan	160		$ 160
Issuance of common stock in connection with employee stock purchase plan , Shares			10,534
Reversal of estimated offering costs	43		$ 43
Stock-based compensation expense	700		700
Foreign currency translation adjustment	16				16
Unrealized gains (losses) on marketable securities, net of tax of $0	(68)				(68)
Net loss	(5,088)					(5,088)
Balance at Sep. 30, 2020	$ 109,550		$ 501,486		$ (16)	$ (391,920)
Balance, Shares at Sep. 30, 2020			27,159,065