Combine the lemongrass, sugar, and water in a small saucepan over high heat, stirring until the sugar is dissolved. Remove from the heat and let steep for 30 minutes. Pour the mixture through a fine mesh sieve and into a small bowl and discard the solids.
Combine the tequila, lemongrass syrup, lime juice, and triple sec in a cocktail shaker filled with ice and shake for about 30 seconds, until cold. Strain into a salt-rimmed glass filled with ice and serve.