Transthoracic echocardiography (TTE) is the first-line tool to evaluate isolated tricuspid regurgitation (TR) but it has limitations and its TR quantification compared with magnetic resonance imaging (MRI) has been studied infrequently. We compared isolated severe TR quantification by TTE against MRI and developed a novel TTE-based algorithm. Isolated TR patients graded severe by TTE and who underwent MRI January 2007 to June 2019 were studied. The TTE and MRI measurements were analyzed by correlation, area under receiver-operative characteristics curve (AUC), and classification and regression tree algorithm of TTE parameters to best identify MRI-derived severe TR (regurgitant volume ≥45 ml and/or fraction ≥50%). A total of 108 of 262 (41%) that were graded as severe TR by TTE also had severe TR by MRI. There were moderate correlations between TTE and MRI in the quantification of TR severity and right atrial size (Pearson r = 0.428 to 0.645) but none to modest correlations between them in right ventricle quantification. The key TTE parameters to identify MRI-derived severe TR in the decision tree regression algorithm were right atrial volume indexed ≥47 ml/m² and effective regurgitant orifice area ≥0.45 cm² and especially if there is right ventricle free wall strain ≥ −9.5%. This novel algorithm has an AUC of 0.76% and 79% agreement to detect severe TR by MRI, which higher than the American Society of Echocardiography criteria with AUC 0.68% and 66% agreement (p = 0.006 and p <0.001, respectively). In conclusion, TTE-derived TR and right atrial quantification had moderate correlation and discrimination of severe TR by MRI, from which a novel TTE algorithm was derived, which had incrementally a higher accuracy than contemporary guidelines’ criteria alone.